why were they attracted to the russian roullete???

Both their psyches were shattered by the experience, Mike less so. Nick got lured back into the game by the sleazy Frenchman, Julien, whose instincts told him Nick had suffered and survived Russian roulette while held captive, and saw in him a moneymaking opportunity. Nick couldn't resist the temptation to revisit the horrors of the game, precisely because it was so horrific. What you see as the reason he wouldn't get into it is exactly the reason he did, perhaps because he had to confirm that his nightmarish experience actually happened. Nick got hooked on heroin and chose that numbing escape, Russian roulette and all, to the reality of returning back home.

Mike, the stronger of the two, was slightly less shattered, but like Nick was drawn in enough to actually attend the roulette games. Mike remained sufficiently anchored to his old life that he attempted getting reintegrated back home, but found it nearly impossible. The best he could do was finally act on his attraction to Nick's girlfriend, Linda, and sneak off with her to motels. Despite the obvious betrayal, I think he did that as much to remain close to Nick as he did out of love or lust for Linda.

One thing I find interesting to ponder is whether Mike, despite his superior inner strength, could have wound up just like Nick had he stumbled upon the Frenchman "Julien" first (whether he did meet him prior to returning to Vietnam to retrieve Nick was unclear). Could Julien have coaxed Mike, too, into playing? Did he actually try, but Mike rejected the advance and was content to be a spectator?

In any case, I think the answer to your question is that the inclination to revisit something life-alteringly awful can be strong. You see this also in Captain Willard from Apocalypse Now, a film that explored the beauty in the horror of war unlike any other.

I'm under impression, oftentimes abused women - if there relationship ends, will again seek out an abusive type of man to be in a relationship with. Seems like they'd run the other way, but people are oftentimes drawn to what they know, what they're used to. If she meets a wonderful man who treats her great, it's uncomfortable for her as it's not the norm.