Suicide not a "sin"

I continually hear about sin, about good vs. evil, and recently about a belief that suicide is a sin.  What is sin exactly?  According to long-held teachings, they are either mortal or venial.  A biggie or a small slip.  It's not enough.  Who put this in motion and decided on the rules, without resorting to a Biblical explanation, vis a vis the Ten Commandments.  No one keeps them and everyone believes they're doing good, so to me it's moot.  My personal belief is, all you need do is love God and be nice to people.  Sounds simplistic I know, yet why shouldn't it be that easy?  But the subject of suicide, it worries me.  If I gauge this by today's thoughts, I'd have to conclude that anyone who persists in actions that they know will result in their death are "committing" suicide, a "sin".  The first person who comes to mind who did that is Jesus.  Did Jesus commit suicide?  He surely didn't stop the events that He knew would lead to His death.  See how tilted it sounds?  And what of the thousands of martyrs who allowed themselves to be killed for lack of saying a few words.  They knew they'd die, yet did nothing.  Are they sinners, did they commit suicide?