Mace was convinced that if he didn't kill Palpatine, the Galaxy was doomed. In which he was mostly correct, though even so it was an extreme thing.
I think people would, in this instance, be curious as to your motivation for killing without warning or challenge a cripple with a slashed open body who was in so much pain he was having to drag himself along the walls to move a few feet, whose weapon was not active and who, if left alive, actually isn't going to make any difference at all as he'll be dead soon anyway, even if he wasn't obviously aready doornailed from his wound.
Other than the hum of Andro's sabre...
(... and actually the noise of Sunda cutting opoen the floor beneath you, but we'll ignore that for dramatic effect...)
... the room is silent. You can hear the painful, laboured breath of Sienar as he drags his face up to yours, Roan.
Then, the breathing stops, and for a moment as he gazes at you, you feel all the rage and hate emanating from Sienar drain away.
"Oh Roan..." he says. "So little time..."
And with that, he falls backwards and lands heavily on the ground. After decades of rot and abuse, the body can take no more; it virtually snaps in two.
Stripped of most of his clothing and exposed lying on the ground, all that is left below you is a shamble of bones and ravaged flesh.
It is hard to believe it was ever alive.
When Jelena levitates Sunda, she is trying desperately to hold her breath against the suffocating fumes of the toxic gas, wrapping part of her robe around her face to assist. As Sunda starts cutting, she uses the Force to spin him slowly so that his twinned lightsaber cut a circular hole in the 'ceiling' above them. Once it is gone, she moves Sunda aside so that it can fall, then levitates him out of the hole. After the Padawan is clear, she will jump out of the death trap herself and gulp in many fresh breaths of air.
She then observes the rotted and unmoving mound of flesh that used to be Sienar.
"It seems that you have given him a release from all his pain and hatred," Jelena says to Roan.
(Well, his Force power was nothing to laught at. He did try to crush Andro with a portion of a building after all. Anyhow, my point was Andro wasn't acting out of evil intentm, or reckless disredard for morals. Anyhow, I see your point.)
"Yes, good ridance, and good to see you two as well. As I was about to tell Roan, we have another problem." Andro takes a deep breath.
"Zaland woke up. We have to find Gallagher, and arrest him... For murder."
"Well, I wouldn't say we've won yet. Now that I realize the whole scope of Sienars plan, it only means that we have much much more work to do in order to prevent him from winning out in the end. Not all battles are ended with Lightsabers my young Padawan. It seems that this battle began with Lightsabers, but will end with words."
"We cannot arrest Gallagher, but let us go talk to him. Andro, fill me in with what you know."
Jelena will proceed to the door and open it.
"Your Master is right," Jelena says to Sunda. "Killing Sienar was not the answer to our problems at this point. But for now, let us see if Gallagher can yet be saved from an awful fate."
She will proceed through the door, lightsaber held cautiously in front of her but unlit.