The big difference is that this is generally voluntary on one party. To use your baseball bat analogy, it's like if a guy said, "alright, it's okay if you hit me with that bat" but then the guy got hit waaaaay harder than he expected. He still consented, so it wouldn't be assault, but it might be attempted murder, manslaughter, or murder, depending on the nature of what went down. While some might consider it assault in the "I wouldn't have initially consented" sense, that would carry a much lower sentence and much weaker connotations than my described scenarios.