But the Romulan on the other faction is still bizarre. Apparently he was never a Republic officer; he's some other form of ally. And he's pretty nonchalant about the fact you just wasted his crew.
Are you talking about "Memory Lane"? If that's the mission you're refering to - the other Romulan captain clearly is an officer in D'tan's Republic, he just happens to picked to work alongside the opposite fation you choose to cooperate with. That's pretty much why you don't kill him, he's a Republic officer just like you.
About the crew, well, your team killed a bunch of (in my case) Klingons and none of them is Romulan, so it's not very far fetched to say that it doesn't overally stress him (we all know what Romulans are like ), especially after you spare his life and could have desintegrated him togehter with his away team.
Because true Starfleet Captains command Exploration Cruisers!
Thank you Cryptic for finally giving me MY SHIP!!!
Yes, the other Romulan states outright that he chose his alliance because he thought that side could help the Republic more, and he thinks you're stupid for choosing the way you did (once he stops thinking you're a terroristic traitor, that is). But once he figures out that someone manipulated you both into a situation where they thought one of you would kill the other, or maybe both would die, he's more than happy to join forces with you to take them down.
And his allies that you wasted? Hey, it's not like they were Romulans or something, right?
"Science teaches us to expect -- demand -- more than just eerie mysteries. What use is a puzzle that can't be solved? Patience is fine, but I'm not going to stop asking the universe to make sense!" - David Brin, "Those Eyes"