Honesty , i felt disappointment coz i faced this position , About what i did : i brokeoff of my relations with them , coz they didn't deserve to be my friends , even they didn't deserve to think of them . so i forgot now when i met them..

hire a hitman/hitwoman :P

i think i'd really feel bad.. i mean.. it's your bestfriend right?? i suppose you've been together for a long time and you've been through a lot.. and that should sorta establish a certain "bond" between you two.. and i think you've put lots of trust in that person.. and suddenly just betraying you would be very rude.. but i think it all would depend on the circumstances.. you should hear also why she/he has betrayed you in the first place.. but initial reaction would be that you would feel bad.. angry..