Loki, of all people, becomes this to his adopted daughter in the fic Mischief, Lies, and Other Hazards of Parenting and its sequel, Thor's Days. In the first he practically bolts to the park where she's having her birthday party when he thinks its being attacked by Dr. Doom (it wasn't), and in the second he almost snaps her scout leader's neck when she's injured due to said scout leader's incompetence (Tony intervened that time). And then in one chapter, Natasha comes to him and makes a hint that Clint might go near the girl. Loki's response is scary.
Loki: I will, however, show no mercy to anyone, and by anyone I mean your dear Agent Barton, who chooses to strike at me through her.
Natasha: So if Clint takes Kara, you'll kill him?
Loki: Has my dear brother ever told you how he and I would sneak into the orchards of the goddess Idunn when we were children? Those apples, which I was so very good at stealing, grant whoever eats them a very long, very resilient life. If your little friend does anything to Kara, I will find a way into Asgard and strip Idunn's orchard bare. I'll cram them down his throat and save enough for you, so you can watch as I torture him for eternity. No matter how much either of you beg me to kill him, he'll live through every length of gut ripped through his flesh, every severed limb, and every drop of acid spilling into his eye. Now is my meaning clear?