Hi, “a tad” means “just a little” of something. For example: “Add a tad of lemon to your coke for a little zest.” “Overkill” describes doing something more than what was necessary to complete a given task. For example, swatting a fly with a garden shovel. Literally, then, “a tad overkill” means to do a little more than is necessary to complete a task; however, here it is sarcastic, because there is really no such thing as “a little” overkill; all overkill is a lot. So, it sounds kind if funny. And when it’s pronounced, one generally draws out the “tad” and raises the pitch to emphasize the sarcasm. I hope that helps!

It's a slang that's used in video games a lot, like doing more damage than necessary to kill an enemy you'd say "Dang, that's a tad overkill", it could also be "That's a tad bit overkill" As the tutor below me pointed out "a tad" means a little bit, and overkill means doing more than necessary, so it's like you're saying "Tad overkill" = "A bit too much", I guess that will help you understand. If you have any more problems with slang and similar things, you can take a look at my profile, or don't, it's a suggestion