Before I go into my “In-Fiction” rant I will put a simple out. The Ultramarine taking an action with the intent to kill his fellow battle-brother, even one who lets his hate guide his every motion without reason, could just be chalked up to the fact that there is the 'crazy aura' going on in the glade, tagged along with the tainted Vespid mineral crystals. (Swamp gas catching the light of Venus) Its an easy out, no one loose their character, though some heavy insanity points are laid out on the ultramarine who will be quarantined and 'treated' for this tainted mind of xeno's sorcery. (Which in the real world is you telling him why it was wrong for him to attack a fellow battle brother with a missile, and the proper use of fellowship and orders... I assume the Ultramarine was the squad leader.)
Now, the long winded answer.
The Black Templar was in the right to do as he did, as would any member of his chapter, unless given 'specific orders' to stay his hand. Though, after all thise he may pray for the addled and alien tainted mind of his brothers who's wills were not as pure as he to hold temporary madness in defending the plague spawn of the foul xeno's.
If he had been off the leash in making his choice to burn the alien larva, and the other space marines wished to stay his hand, all they simply needed was to intervene with words and simple physical imposition. (Example: “Hold.” The space wolf stepped forward to press down his brothers readied arm to stay its killing flame. “What honour is there brother in killing mere whelps, surely you hold yourself higher they such glory-less slaughter.” The Ultramarine came behind in support of his brother wolf. “Aye, Astarties do not stain their hands with such blood. That barbarous work is left for the broken shells of mortal men and the cold guns of the navy.”)
This might not have convinced the Black Templar to stay his hand, but it would open a nice moment for roleplay, and if needed, an opposed command/intimidation roll to see who backs down. It should have never come to blows and that's what all the players need to remember.
Though what's done is done and its up to the players what happens next.
If the game ended there and the next game is the conclusion, it would be up to the squad leader, or if he was involved, the next in the chain of command who is impartial, to solve this issue. Unless their watch captain already knows what happened they can solve this internally, blaming the madness or whatever else that can be made up to justify not simply reporting this back up the chain of command where the heavy hammer can fall. Resolving his within the kill-team should have them loose all their cohesion, save the MOS on the mediators fellowship roll. Insanity and ingame rivalry should also be present, and also become a roleplaying plot for a shameful secret the team carries with them.
If its to late for that, or one of the Kill-team acts on his own to tell of what has transpired there will be a full investigation into the ultramarine, the space wolf, the Black Templar, and the kill team as a whole to see where the weakness lays and all will be punished, though not equally. The squad leader would be punished with some kind of penance for allowing such a break down of discipline to transpire, brothers that stood by while this happen would also serve penance for their lack of initiative to intervene in disaster.
The whole point is if this gets past the Kill-team heads will roll. Anything that comes from up on high will come down with great force and weight and will be done swift and without mercy or fanfare. The Deathwatch will keep the inquisition out of the loop and handle the matter itself to avoid shame and scandal, but if the holy ordos is apart of the loop it will be drawn out and the tools of confession will be sharpened and all in the Kill-team will pay for the incident, no matter how innocent they appear. “Innocence proves nothing” and they all failed.
Then comes the meat of the issue, the main three offenders. Apply three penalties if the incident goes public. Permanent loss of Cohesion while working together unless they can find forgiveness with each other. A loss in Renown due to their actions that transpired to such a sad end. As well as wheather the character is now filled with hate or guilt and what he is going to do about such emotion.
Don't worry about the severity the penalization on the kill-team affects their performance in mission, like its been said, this is a big deal for space marines and they should never want to do it again. Their a puppy that's been trained to go outside and has crapped all over the carpet... get the pepper out and rub their nose in it.