Repent!

Here's a little poser to think about over the weekend. Below is a typical explanation from a devout Muslim of the system of reward and punishment in Islam :
This mercy [Allah's] is such that if a man sins all his life and sincerely repents prior to the appearance of his death-signs, all of his previous sins will be forgiven and he will go to paradise. I personally do not see a problem with the punishment of oppressors and murderers, nor with the provision of a reward in the afterlife to people who struggled for truth and justice in this life. In fact, it would be terrible to think that people who got away with major crimes in this life should never be called to account for their misdeeds. We absolutely need a Day of Judgement if a just God exists.
Can Muslims please explain this? One the one hand the above seems to be suggesting that a terrible sinner ("a man sins all his life") can get into Paradise by "sincerely repenting". But on the other it defends the appallingly sadistic punishments Allah delights in inflicting upon "sinners" by saying how awful it would be if people "got away with major crimes in this life". And yet the terrible sinner who repents does just that, doesn't he? 
So in Islam it is possible for a chap like me or Steven Fry or David Attenborough  to get roasted in hell for eternity for dying before we "saw the light" and believed in God, whilst a  murderer or rapist who sincerely repents gets to shag 80 foot tall tall, miraculously regenerating virgins with see-though limbs whilst drinking magic wine.
I do try to understand your religion, but I keep coming across these logical hurdles. 
I shall be seeing my Muslim friend shortly. I shall ask him his thoughts and report back...