One question, then, would be - did the suspect understand that the robot was going to be used to kill him? Was this announced? Was that threat obvious to him?

There is also a question of time pressure: if police are just camped outside my door telling me to drop my weapon I will be less inclined to do so because the threat (for both of us) is not yet imminent.

If they instead say, "we are coming in. Drop your weapon or we will shoot" the threat is now imminent for both parties.

In this situation, the police are using deadly force without (a clear) imminent threat. That feels like a big red line to relax.