sin -- and its levels and consequences:

God has provided -- through His Universe's fundamental laws/principles and through Christ's teachings and through the Holy Ghost's grace/communication/inspiration -- optimal pathways to life, love, truth, beauty, virtue, and the wisdom to appreciate God & man's relationship to Him.  When man deviates from these optimal pathways, and when he induces various degrees of error & inefficiency [non-perfect completion, or delay in completion, of the Lord's domain:  "If you would be perfect, come follow Me."] to mar their perfection through 'sin' [murder; theft & induced poverty & induced squallor & induced non-fulfillment; greed & unreasonable non-cooporation; irresponsibility; adultery; idolatry; contempt for learning; distortive/disruptive 'false' pride; lieing & injustice; etc.], then he increases the negatives of this world [death, ignorance, accident, illness, war, etc.] and consequently bears/identifies-to-himself these sins/effects as his life's responsibilities/fruits, and makes them an integral part of his personal identity/make-up -- unless removed via sincere repentance/repair [if no repair, almost certain was no sincerity of repentance -- or immediately recommited the sin by acceptance of its ongoing negative fruits which he was in position to ameliorate].

The level of one's guilt depends on two factors:
      1.    Seriousness of the sin/inefficiency-to-life-advancement that is imposed upon the world [and what has effect upon the sinner, also has effect upon world]:
             a.    minor  [often done unawares, through own ignorance or bodily deficiency ('sin' of tragic -- original sin -- incompleteness in the world, but which sin the world's fundamental principles & God's grace allow us to recognize & to improve & to eventually overcome)]; such as blindly rushing over log to tear clothes or bruise leg; or carelessly putting partly rotten apple in with other good apples and so damaging many of them; or singing off-key; or over-eating; or losing proper control of temper; etc.;
             b.    major;  such as driving vehicle when over-tired [and so with increased potential to miss stop sign, and so to cause harm or even death]; or any irresponsible act that kills or cripples; or some involvement in hoarding, so as to impoverish and so deprive or undermine someone of education or of health or of family ties or of freedom to act or of good reputation; or some act of pollution that causes illness or pain; etc..
      2.    Deliberateness of the sin/inefficiency-to-life-advancement that is imposed into the universe:
             such as being aware that are driving totally exhausted or even drunk, but not caring if injure or kill someone, including oneself; lieing to another's detriment; deliberately being sloppy in sterilizing items used in canning tomato sauce, and so endangering many lives via food poisoning; willfully & needlessly imposing pain or even death upon some sentient -- feeling & conscious -- being; supporting precipitous initiation of violence or war.

The terms venial & mortal have often been used to describe various categories or gravities of sin, with mortal being reserved for sins done deliberately & done causing major harm, and so resulting in loss of all the soul's sanctifying grace -- i.e. all grace that constitutes true life; ergo, one's soul is totally dead.
This is somewhat misleading, because total death of soul constitutes virtual infinite totality of opposition to God, and while this life of grace to the soul is loving gift from God and so not something we claim by 'right', nevertheless, in trying our best to imperfectly evaluate the so-called 'mechanism' of its operation, we must always keep in mind that man is not 'infinite' in anything that he does or doesn't do, and that includes his sins; but that is not to say that man cannot eventually come to such total opposition to all that is wholesome & good in universe, to all that God stands for [indeed, Christ has made clear that the number of these lost souls is quite great]; but it would usually [singular exceptions via great sins committed with total personal commitment to the sin might qualify, especially if that commitment to evil lasted through some various personal readjustments of focus & appreciation, and over some significant time period (which
time period could be within day or hour or even less, because intensity & degree of commitment are related to time -- sequencation -- itself, and so can develop in one hour what otherwise might require years:  and God will never stand in way of core-essenced free-willed rejection of Him)] be more of life-long process of constant re-affirmation of the essence of the sin/evil/inefficiency-to-life-advancement, or even constant non-affirmation of anything ["If you are
lukewarm, I shall vomit you out of My mouth." -- and clearly out of heaven: to hell!!? -- or?, possibly?, to eternal naturalistic limboic intermediate state?], and especially including non-affirmation of one of the virtuous aspects of the Godhead [Matt. 25:43  "I was a stranger, and you never made Me welcome."], and consequently rejection of God and of the opportunities to heavenly, true, and eternal life essences which He has presented to one throughout one's lifetime.

One could say that sin is a scale starting with the most 'minorly venial' at left, and with growth in seriousness to most 'majorly venial' [but with greatest potential for mortality/soul-death] at right, and that it is the degree of commitment to some serious sin and to its effects [which one never truly rejects and tries to repair],
and the time spent in that commitment which are what -- over period of significant part of one's life -- finally bring you to soul-death/mortality-of-sin:  the greater seriousness of the sin [including the sin of inaction], the sooner the arrival at mortality of soul-death [soul-death = still aware soul stripped of all sanctifying-grace/God-essence-of-eternal-life].
The sanctifying grace of eternal life is gift from God -- not owed to us -- and it is our responsibility to pick it up, and to internalize it to our personality; and if one doesn't do so over lifetime of opportunities, then have only self to blame for exclusion from heaven.

To somewhat repeat this serious matter, but in slightly different form, sin = diversion away from God's characteristics/grace with their positive orientation of life-truth-beauty, and with this diversion being marked by ever greater inefficiencies-to-life-advancement in proportion to the degree of seriousness of said sin/diversion, and with the ultimate diversion/inefficiency-to-life-advancement being 'death' ["the wages of sin are death"]. 
Each person's sins/errors impose some inefficiency-to-life-truth-beauty upon whatever it effects, whether mostly just in own life, or substantially in life of community & entire world [to kill & injure others; to cheat & destroy; to lie & impose mistakes in understanding/appreciation/operation -- which things rob even the virtuous-just of legitimate prospects of peace & enlightenment & prosperity, and so of variety of grace-facet developmentsof their soul-identity].