The most effective way for the US and EU to defuse any political fallout over the rogue actions of a bunch of scientists is to call it exactly that in public. Admit to their errors in judgement in choosing (as yet unknown) members of the Mars teams who could go on to commit murder, and state that the number one priority is now not only to rescue the innocent, but also to arrest and prosecute the guilty.
To that end, the 12 crew members who were scheduled to go out on the next resupply mission would be withdrawn, and instead a 10-strong team of military- and police- trained investigators, all with combat experience in either a civilian or military conflict, as well as experience in rescue operations, drawn equally from both the US and EU and ideally having professional experience and friendships with one-another, would be substituted. All of the new/replacement scientific gear would be removed and substituted with military and investigatory equipment for the use of the investigators to bring the perpetrators of this "unwarranted violence" to justice. The last two positions would be specially selected psychologists, again from both the US and EU, who would be working to determine what caused the build-up of tensions that led to the open conflict, and what could be done to prevent them from accumulating in future missions.
Despite both the US and the EU wanting the missions to continue, they must publically speak as if cancelling the missions entirely is a high probability - which in fact it is. Until the rogue elements in both facilities can be identified and arrested, the missions are in jeopardy. The best option from the Earth-siders point of view may be to entirely replace the existing Mars teams with entirely new teams.
It is unlikely that the perpetrators could escape justice for long once the resupply ship arrives. While they know the environment better, Mars is still a hostile environment, and environment suits have limited supplies. They would be forced to either surrender or attempt to fight off the investigators from whatever bolthole they could find, and the investigators would have vastly superior combat skills, likely backed up by armoured environment gear that the rogue scientists would have trouble dealing with.
In any event, it is one thing (as most likely happened) to pop an enemy environment dome using a drone, and entirely another thing to face multiple trained soldiers and police officers each armed with a gun selected and loaded to deal with precisely the sort of protection the scientists may be able to muster. In all likelihood, the scientists will surrender immediately rather than fight, and the probable fate of the first one who doesn't will encourage the rest to choose a wiser course of action.
Once the scientists have all been rounded up and placed in protective custody, and all the bodies identified, or at least accounted for, the investigators will begin the investigatory side of the operation, attempting to determine who did what.
At the very least, the suspects will be returned to Earth for trial on charges including murder and terrorism, and the investigators may conclude that the entire scientific team needs to be replaced eventually.
Regardless, the suspects will be shipped back to Earth in custody, while the investigators remain to fulfil a police role in order to keep the peace between those who remain.
Back on Earth, after debates over jurisdiction in which it is suggested that each body prosecute its own nationals, the perpetrators will appear in highly publicised trials in the Hague, which, based on the Outer Space Treaty has jurisdiction over crimes committed in space. See also: https://space.stackexchange.com/questions/683/jurisdiction-over-crime-in-space.
Since the crimes in question are murder and terrorism, there will be no question as to whether the alleged actions are worthy of prosecution or not, as both the US and the EU have laws concerning the accused's actions.
As to the outcome of the trial... who can say?