See to me, Lex isn't the kind of guy who gives up. And killing yourself is the ultimate form of giving up.
Lex has probably got so many lawers they're coming out of his a... um... nose. I think it would be very likely that someone as revered as Lex would be able to eventually wiggle out of any major charges. Just think about how many CEOs in our world that have gotten away with everything because they're so big and powerful. So to me it seemed kind of silly that he was willing to just give up on any hope of getting out of the situation. Like LabRat said, I think he would have some kind of back-up plan if things were to fall through for him. Yeah, he's got an ego, but he never got too cocky so as not to prepare for the worst possibility. It wasn't his ego that was his downfall, it was his obsession with Lois.
