Lucidstuck II

Why not observe first and then act instead of act first and deal with the consequences?

Staying here to see if the guards in the front do anything is an action too, and it can have consequences. It could be quite inconvenient for us to stick around until the guards discover that the patrol is no longer patrolling, since they might start searching the area or telling other guards.

Getting rid of the guards in the front would leave us with no-one who can discover our murders. If we can do it, it'd be safer.

Taking a moment to observe their patterns is by far the best way to go for a variety of reasons, including the fact that we won't be able to kill both of them fast enough to remain unnoticed. We have a crossbow, they're not known for having the greatest rate of fire.

Assuming the guards we've already killed would make a full circle around the building (which they might not, in which case we're golden), since we know they don't have radios it would mean the remaining two guards would check on them in person. This means they'd most likely separate so that one of them could keep guarding the doors, while the other goes to search. THAT would be our best time to strike, then go around and follow up on the other one.

Even if that's not the case, we'd need to observe them to find an opening to give us enough time to kill them both without alerting them.