Depends on what happened. If he ran into something while off the road you would probably bear some of the blame for your deceptive move. On the other hand if he runs into you then he would be at fault.
I know how you feel about tailgaters. They do that to me all the time. However, I also do it. When im a rush and driving fast and passing sometimes I can approach too fast and end up getting closer than I want to. I usually back off. So sometimes the tailgating is just a mistake, especially considering the move you pulled there :(
The folks I hate the most are the ones that are entering traffic from the on-ramp and think that means they are supposed to zoom past everybody until the ramp runs out then take the fking pole position! WTH is that? \o/ Especially when I was also on the on-ramp and just happened to merge in first chance I got like a civilized driver.