How long do the tantrums last? Do they Stop as soon as he gets his way?
I think you need to take a more structured approach to his having screen time. It shouldn't be whenever he feels like it, he should earn (for example) 1 hour of screen time by doing something desirable in the household, or playing at a sport outside, or doing something active. He's old enough to earn his way in.
I realize an iPad, like a TV, is a useful babysitter, and since you have mentioned that the person he was angry with over the lack of the iPad is his grandmother, is she his babysitter or caregiver? A kid should not spend more than an hour a day onscreen, and the fact that it has come to tantrums suggests he's had a lot more access than that. If he is on the iPad without anyone supervising what he is doing, it's likely he has watched the kind of stuff that gives the vocabulary or imagery of killing, to express anger or frustration. Not to mention that unsupervised screentime doesn't teach anything about discipline -- flick a switch and never need to tolerate boredom or have to think of something else to do to entertain oneself.
Is he getting any physical exercise when he is with grandma, or reading, or doing art, or even just going out and running errands with her? Anything where he has to engage his brain, tolerate frustration, and be patient? Or mostly when he is there, is he on the iPad? I'd take it away entirely if that is the case, and don't let him have it and tell Grandma she needs to find another way to keep him quiet when he's with her.
Sounds like this 6 year old needs to be disciplined!