The answer is simple. It is known that a Ganev is worse than a Gazlen. Common sense dictates that a shoplifter should be less-severely punished than an armed bank robber. However, the Torah doesn’t [always] work with emotions. A Ganev demonstrates that he isn’t G-d fearing – he hides from people but not from Hakadosh Baruch Hu. A Gazlen is a desperate individual who simply needs money. He places his pressing needs before anxiety, therefore being better off.
Same here. Doing harm to oneself is equal to harming someone else. One has no right to do damage to his body. Therefore one who murders someone out of temper or panic is better off than someone who kills himself to ‘get off the hook’. A murderer simply forgets what he is doing at the moment and isn’t necessarily doing so in defiance of G-d , whereas a suicidal person displays his ignorance and disbelief in the world-to-come. This makes the latter a Kofer in Hakadosh Baruch Hu and loses the benefit of forgiveness in the world to come. The Kevura and Shiva cleans the Neshama from stains gathered over the years and enable it access to the world-to-come. A non-believer doesn’t deserve that benefit.
It should be noted that many Achronim (including the Chasam Sofer) basically scrap this Halacha (and claim that it doesn’t apply nowadays) given that one usually regrets his actions before dying.