Shirou: S-Sab-
Kamui: My Master!
Illya: Good Boy Berserker! Don't kill him though, I want him to suffer more!
Garmr: Garmr won't let Shirou hurt Master!

Garmr would be a very dangerous Servant in the hands of master such as Illya. Not because of his monstrous strength and speed but because of his near blind devotion to his Master whether if they're good or evil. Now I don't think Illya would abuse Garmr (though she might get annoyed by his stupidity), she
would definitely be a bad influence on the guard dog. If she ordered him to hurt or kill anyone, Garmr won't hesitate to follow it, even if the target's completely innocent or if it's someone he knows. Only Illya's morals are Garmr's morals, her choices are his choices. Something that freaks out even Horkeukamui.

Illyasviel von Einzbern by Tamahana © Type Moon Fate/Stay Night Fate/Zero
Emiya Shirou By Good Luck © Type Moon, Fate/stay night
Garmr by siba81715928 © Lifewonders Tokyo Afterschool Summoners
Horkeu Kamui SachaShan