Is he now an alien? Has been body been taken over by the bad-guys. Would the old Skinner kill Krychak like that in cold blood? Would the old Skinner enjoy it so?

hell yes, Skinner would have enjoyed it that much. I don't think anyone on that show hated Krycek more than Skinner....not even Mulder (Krycek killed his father) or Scully (didn't Krycek kill her sister too?)....
Krycek held Skinner's life in his hands anyway and could kill him anytime he wanted to since thos tiny ceno-bots or whatever are in his blood and Krycek posesses the thing that activates them.
I don't think Skinner is a "replacement", but that's just my opinion....

Yeah, everyone hates Krycek. Skinner's not an alien.

In a heartbeat. He has it in him to do it and for all the reasons that garmonbozia stated. Plus I think one other reason, he wanted Skinner to kill Scully's baby.

I happen to think that Kirsch is an alien.

As long as Krycek was alive.... Skinner would be his bitch.

Skinner hasd taken his fair share of licks from krychek, but Mulder ultmatleu became his arch nemesis, mulder should have killed him in season 3, and again in the russain camp, but c'est la vie

What if Krycek was a replacement too and he's not really dead??

Hell yes skinner has it in him. like mentioned above. skinner hated Krycek. worse then anyone could imagen.

Hell Yeah man.

I think of it this way. Krychek had those little nano-machines that were running all threw his body, and he controlled them witht he "pronto"/"palm" unit.

He could not stand him. I was thinking the same thing. Then I was like nah....:)