It's a slight misswording from the translator part, the raw said something like "That's a pity.."/"Such a shame..", "I feel sorry for you, Amada". Basically the type of things you said when you saw someone messes up.
The reason assassins were mostly depicted in full black attire (usually a little bluish) is because they work in the dark (at night).
If the author want to make them do the ambush in broad daylight, he shouldn't have made them done a full black and just go with the usual bandit attire (or...
The energy source, that's prolly gonna be the key to defeating this machine, no machine can go on properly forever without any maintenance, so it's either that or they just gonna seal it somehow.
Reason:
1. Kobachi doesn't really understand love, personal distance, and common sense in general.
2. Tsubakibeni doesn't know about Shinonome's feelings.
3. Kobachi is basically blackmailed because she absolutely doesn't want her other classmates to know that she's still in highschool.
4...
Correction:
The first few hit "can" be considered as self-defense, but any subsequent hit is not (because there's no further threat, since they're clearly backing up out of fear), and it would simply fall under "retaliation" which would also get you in trouble.