Alec would lose his fucking shit <3 if you ever want to see Alec go full ballistic, that would do it. Scorched earth on whoever did it if he can find them. As for how he would feel...honestly, mostly angry and deeply upset that he wasn't able to prevent it. Powerless, in a sense, which would make him all the more possessive of Isabelle and determined to seize and exercise power where he can. There would also be a teeny little bit of grief in there that he would force to present as yet more anger, or else in intimate moments as tenderness with Isabelle. He would never fully get over it.