To be honest, I feel like he has every right to be upset with her. Can you imagine what that feels like? His fiance took a bullet for her cop ex-boyfriend in the middle of a raid on their wedding to arrest him.  But let’s look at this a little deeper while we are here, shall we?

Accidentally shooting the person you are marrying is already traumatizing enough. It was bad. 

Since Moonflower’s death, Altair had already been acting pretty strangely. Sirius isn’t stupid. He noticed she was acting strangely. The whole pool party incident when Altair stormed off after seeing Ume with a new girl makes it pretty clear she still had feelings for him.

Let’s be real here. Sirius’s crimes aren’t petty. We are talking the murder and torture of many berries here. (It’s a lot. 20 years is a long time to spend as an “enforcer” in that business.) The consequences of him being caught are literal life and death. It was the very reason why it took Altair years to even consider being in an “official” relationship with him.

Lupin was quite a bit behind. Had he shot Ume and ran, chances are he probably would have gotten away. Instead, he was forced to face the horror of shooting the woman he loved, that he changed his whole life for. Talk about trauma.

This line here pretty much says it all: 

A life for a life. That’s what that moment boiled down to. I didn’t choose his…

We are often more true to ourselves in the most desperate of times than any other. Do I choose a life without Ume or a life without Sirius? When backed against the wall Altair chose Ume without hesitation.