To play devil's advocate for what Steele did (not agreeing with what he did, just trying to understand Steele's justification):
Doesn't the Geneva Convention allow for the execution of spies/those convicted of espionage? Steele might say what the Elder was doing to an extent (trying to break up the GTVA). If the Elder was trying to conduct secret negotiations with the Vasudans, is it that surprising that Steele would try to stop the Elder from making it?
Or to put it another way: what should Steele/GTVA have done when they found out the UEF was trying to break up the GTVA via secret talks? Should they have just tried to capture the Elder instead?
First off, taking the Elder captive was out of the question. Steele tried (and apperently succeded) in making it look as if the UEF was trying to manipulate the Vasudans with the help of the Gefs. He made the Vasudans believe the assasination was just another plot by the Elders and that the video recordings were just forgeries.
If he then sends out some kind of demand to the UEF, that they have to fullfill, to get their Elder back, it would ruin that deception and probably cause a major political crisis between the Terrans and Vasudans.
As for the bombing of Luna, that is one thing we can't blame on Steele. That attack was the last order that Admiral Severanti gave in Sol (probably to try and save what was left of his career). If they really wanted to, the GTVA could probably blame the whole attack on him and say it was never officially endorsed.