Buddhists/Killed and abused animal as a kid
When I was around 7 a rabbit bit me and I got angry and intentionally threw it repeatedly and violently on the floor until it died. I got upset immediately when I notice it died. (Also I would like to mention I was not treated well as a kid). The incident left a heavy impression on me. Recently I had a dream that my grandma's spirit is waiting for me but I can't join her because I was turning into a rabbit. I've always regretted about killing this rabbit and especially so after this dream. Can you tell me if this dream is going to come true and if there is anything I an do to overcome this negative Karma?
The dream sounds like a very good sign. It means that deep down you truly and genuinely regret what you did. In Buddhist teachings, that is the first and most essential step to purifying past bad actions. A commitment not to do the same thing again is another key point, and it sounds as if you have made that. If you want to go further, you can, according to the teachings, when possible, try to put right the harm that you did. In this case, that is obviously impossible, but you can take your regret as an inspiration to do something in the opposite direction. Support animal charities, support animal rescue organisations, and so on.
If you are religiously inclined, you can also do "religious" things like prayers, purification mantras, and so on, but which of those might be effective depends on your background, understanding and religious connections. There are plenty of specific Buddhist things to do, but when I say specific, I mean they are specific to particular traditions within Buddhism. I don't know whether you are in that position at all, and it's not the kind of thing you can just "add on" artificially. But genuine regret, determination not to repeat it, and working to stop cruelty and save lives are the heart of the matter.