The whole killing Alaric thing can be very beneficial for Tiffany, for one, she can get out of the marriage w/o refusing her dad's plans for her while also spitting in his face w/o him even realising it, she can also play the role of a distraught widow to maybe use as a cover for doing something else in the meanwhile, nobody would expect someone who is depressed due to her future husband passing away to actually be plotting something in the background.
It works even better as a tool to facilitate S.I.N's plans, it could work either as a test for the effectiveness of the chimeras or a show of power from S.I.N to send a message like: "Look, we can kill whoever we want, whenever we want, in front of some of your strongest people, and you have nothing to do against it", which also happens to be in front of most of the most powerful people in the world, who have front row seats of how H.E.R.O can't protect them, or even worse won't protect them, I mean, I'm sure a ton of people could see the total indifference on the captain's faces when he was killed.
It could all also serve as a distraction for an even greater plan, by driving everyone's attention to this newly formed enemy faction who doesn't even seem to be able to compare to H.E.R.O, what tells us that the monsters Ella works with aren't doing something while H.E.R.O is busy crushing S.I.N? They could be handling H.E.R.O a free win in a battle and feeding a false sense of superiority just to guarantee winning the war.
My point is, there may not even be a reason to kill Alaric in specific, but maybe we shouldn't be looking at him, and instead at the whole situation that can come from killing him right there in front of everyone, be it taunting H.E.R.O in order to bait them to a carefully constructed trap, growing further unrest amongst the people in power regarding H.E.R.O, just a way to test the water of what they can getaway with, etc. There's a ton of reasons why they might want to kill him, not because of Alaric himself, but because of the perfect situation that Tifanny's wedding provides (Assuming Tifanny IS in fact working with Ella and/or S.I.N, otherwise yeah, we would have to look at the whole situation in a different lense).
