Continued...
You also suggest that this hypothetical murderer should be shown leniency due to his age. This flies in the face of all Christian teachings. You've just asked if a murderer should be forgiven his sins and you appear to think that he can and should simply because he happens to now be old. But the Bible (both old and new flavours) is quite clear on this matter – kill, and you're going to Hell. Are you saying that the Bible's is wrong on this matter?
You also say, “... If you don't accept that breaking the law of your heart is a sin (A transgression of a religious or moral law, especially when deliberate) then you have no defence on judgement day just as you won't stand a chance in court when you break man's laws...”
This makes an almighty assumption (pun intended) that there will be a 'judgement day'. Which, of course, there won't be. Oh, and your question, “... What's the difference between a lifetime and eternity?”, is a very easy question to answer – (a) 'lifetime' is one's mortal life, measured from birth to death; (b) 'eternity' means from the very beginnings of time (approximately, 14 billion years ago) to an indeterminate point in the future. Since you, I and everyone else is subject to condition (a), I wouldn't worry too much about condition (b).