Suicide breaks the "strive to be good, fail, repent, and strive again" cycle at Christianity's core. Murder also breaks it, but for other people. And murder is forbidden in the Ten Commandments. That said, I think the idea that suicides go to Hell is rubbish.
We have still had problematic interpretations of the implications of paradise after death. During an early crusade against heretics (who were actually only country Christians who hadn't caught up with the times), crusaders faced the dilemma of how to know heretics from true believers. Their solution? "Kill them all. God knows his own." Fortunately, we stopped doing that several centuries ago, after we agreed mass murder wasn't very Christ-like of us.