Betrayal

I don't know why, but I have this urge to betray one of my freinds. I have never experienced being a bad person. And I think thats why, but im not sure.

See, I'm his best freind, and I have intentions of dating his ex-girlfreind. He says that he wouldnt care, but he doesnt knwo I like her.

Thoughts

It can't possibly be betrayal because he's explicitly stated he wouldn't mind this and you have to reason to believe it would harm his interests in any way other than possibly making him uncomfortable which he has already assured you it wouldn't. The only reason it would be a betrayal is if you genuinely believe that they belong together, that it is the best thing for them to both be together and that you dating her would seriously make that any less likely. After all if she doesn't date you it will probably be some one else other than your friend.

If you dating her does make him uncomfortable then it is he who has betrayed him self by lying to you about his true feelings and probably to him self as well.

You can go for it if he's found someone else. If he hasn't perhaps it's not a good idea because people always say it won't bother them but it's all lies and really deep down somewhere in there he does and these lies deep inside are the worst type of evil. Mostly because they just hate you and behind their shadow of lies they really don't know why.
On the one hand you can date her and things will be confusing for a while or you can not and take the safe route out. In all honesty it would probably be really fun just to see what happens.

if kevin smith has taught us nothing else it is that the only way to settle things of this nature is through, ultimately, the threesome. when you propose this admittedly unconventional solution remember, approach them not with shame and trepidation but rather with the swagger of a revolutionary in soulful congruence with his convictions. trust me, its fool proof.