Out-of-game communication.

...
1. Indeed, it's a blunder, but dwarves were fighting, and elves were also there, meaning they were in the city.

2. They pushed Inferno there and then slammed the magically sealed gate shut.

3. In desperation, the demons went into a fighting frenzy; their main goal was to take as much as possible with them, and if necromancers attacked, it meant they were also enemies.

4. Tengro was busy with other things. Naturally, he knew and saw all this, but he had something more important to do; he was performing a ritual.