Erm, okay... Yes, Stranger's secret was that he was a Steef, and he did look bipedal when he was actually a quadruped. However, he didn't want his hind legs removed (I doubt only two of his scrawny legs could support his body weight). If you look at the de-pantsing scene (where Stranger is revealed as a Steef), we see the Doc's drafts of the operation that he gave Stranger earlier. It looks more like he was going to have his legs merged together (maybe sewn).
We're not actually sure
why he wanted the operation, what is most likely is that he wanted to hide he was a Steef so he wouldn't be hunted down and his head used as a wall trophie like all the other Steef were.
Sekto's host body was an old Steef that had had it's horns removed so that Sekto could sit atop his head - this old Steef dies at the end of game. But I doubt Sekto wanted to use Stranger as his next host, although that might have been on his mind. I think though that he just wanted to eredicate all the Steef...
Anyway, hopes this helps.
