That is incorrect. First off, there is only 1 white gang in any prison. There arent enough white people to even fill the one white gang. The AB is vastly outnumbered by the black and hispanic gangs in every prison in the US. Every white person has to stick together in prison.
Also, you never have to kill to get into any prison gang. There are different roles that you can take when you join (you are given a choice believe it or not), but very few people are "enforcers" who do the actual killings. Most people do things like carry shanks for the enforcers. Your cost for having the gangs protection is doing shit like that, but if you get caught with a shank, they can sentence you to several more years, so its a super sketchy job, but you have no choice if you want the gangs protection.
Yes you could say "im not joining", but all the other gangs will know that you arent protected and they will prey on you instantly. It isnt all rape either. Rape is more rare than you think in prison. What they will do is take all your shit. Your blankets, pillow, shoes, stuff like that. Forget about having commissary items, they will take all that, and when someone is in a bad mood, they can always take their aggression out on you because, you arent protected by a gang.