Aleksei is...well, let's just say his feelings for Fiona aren't...

Ugh. My brain wanted to say "brotherly." Ugh. I feel dirty now...

But yeah...Aleksei would literally kill for her. I think he'd be happy if Jacek never came back into the picture. Remember he said," I could love you, if your eyes weren't so lifeless," the first time they met. And now they've gotten to know each other better (in more ways than one!), and he's seen how deep her love of his woods goes.

The woods are him and he is the woods...which in turn means that as she loves the woods...well, she has a love for him too. And the woods...they love her in return, in their own way. Which...transfers to him, I think.

I dunno. But I think that's why, even in his weakened state, he was able to knock Jeremy's brains cold.