Could have been killed, sitting in a cell now, could have escaped, et al. It would be pretty easy for any of those to have happened.
Even if Amon actually killed him, that does not make Amon a monster. Kuvira killed Hiroshi Sato and no one has ever said that Kuvira is a monster. So, if Kuvira is not a monster, then Amon is not a monster too. If Amon is a monster, then Kuvira is also a monster.
You motherf*cker.
I believe his is alive as he is highly resilent to pain, note getting wacked by Naga, and Amon never killed anyone, also he might still be on the loss
Whenever I watch that scene, I think of the same one in The Empire Strikes Back, when Vader Force-chokes his lieutenant.
Imagine that scene in LoK, but Amon says "You have failed me fore the last time, Lieutenant."
Was he going to kill Korra and Mako? The answer to your question depends entirely on this, because if he can't have witnesses, then he should kill both of them. I don't remember what was going on, but i'm pretty sure he could kill them without taking their bending, so i don't think he planned to kill them. I could be wrong though, been some time since i watched it.
I think he only planned to take their bending. He had his fake scar as "proof" that Korra was lying, so why would anyone have believed them over him once they were taken prisoner? Plus, as you said, there's not much reason to take their bending if he's just going to kill her anyway.
The Lieutenant is a more difficult question, because as 2nd in command, he would have a lot of credibility with the Equalists. If Amon didn't decide to kill him right there, I don't know what he planned to do about him, or even if he had a plan. It's entirely possible that he couldn't bring himself to kill his lieutenant, even if it would be the pragmatic solution. But the only way we'll ever find out is if Mike & Bryan ever see fit to confirm what the Hell happened to him.