Cheating.
One of the most worst things you could ever do to someone. If you're not happy with who you're with, than leave them. There isn't a use staying around when you don't feel the same way anymore. Don't think about how much it's going to hurt them, of course it's going to hurt them. But cheating on them would be way worse than just breaking up with them. By breaking up with someone you're giving them time, time to get over you. By staying around just because you don't want to cause them pain will just give them false hope. Causing them even more pain. As each day does go by you're only making them suffer more and more as they fall even harder for you with each day. If you're not happy, then leave. It's as simple as that. There is no excuse for cheating on someone. If you did it, then you did it. The End. Leave while they're still capable of moving on, don't leave it too late. You never know what could happen to them if you've been there for them too long. They start to lean on you, you become a part of them. They make you their everything and once you leave, they'll be left with nothing. So leave, while they still have something...
23chars