When you voice or play a character, it's understandable to feel a sense of ownership over them and new interpretations of said character may not always match up with your own personal taste.
It's been that way with Bond, Batman, Superman, The Joker and so on for so many years. Most actors will note the differences and probably chuckle about how the new one's better than their take on it. However, it really does show just how different Zack Snyder's Batman was that quite a few people have had a strong reaction to it.
Although the name Kevin Conroy might not be immediately familiar to most, his voice definitely should be. From 1992 to 1995, Conroy voiced Batman in the hugely successful, critically-acclaimed Animated Series. Since then, Conroy's voiced Batman in the Arkham series of videogames and, most recently, in Batman: The Killing Joke.
In a recent interview with IGN, Conroy was asked about Zack Snyder's take on Batman and wasn't exactly complimentary about it. "Personally I love the fact that Batman – in the stories I’ve done, and the way he’s been rendered by Bruce Timm and Paul Dini, the people I’ve worked with most closely - he never kills anybody. He doesn’t cross that line. Batman is not a killer... In the most recent live action movie, that seems to have been a line that was crossed and it’s not one I’m particularly comfortable with."
He wasn't the only one, of course. A lot of people took issue with the fact that Batman was now killing people willy-nilly - something that Christopher Nolan's version and many, many years of comic-book writing had taken great pains to precisely explain why he didn't kill people.
Overturning years upon years of story for the sake of perceived realism was always going to p*** people off, so why shouldn't it be the same of people who actually played the character?