When I as a DM have an opportunity to make a major change to a character, such as character death, conversion to a villain or non-villainous NPC, etc, I will bring up the issue with the player. It means you can't surprise them, but by asking their opinion and inviting their input on the idea, you can not only find out whether they will accept/enjoy the change, but also get additional ideas for ways to improve the change.
A few examples: A player was playing twins, one of whom was rather... homicidal. I realized that one of my NPCs would very much want that twin on his team. So I spoke to the player privately between sessions, to see how he would feel and how his character might react to a proposition from the NPC. He was thrilled by the idea, offering several suggestions for how his character could work as a villain from within the party for a time before converting completely. His character ended up becoming a miniboss for heroic tier.
Another player ended up being the Bearer of the Holy MacGuffin for the plot - the character responsible for carrying the items that the villains wanted. I warned him (both in and out of game) that this made him a target for the villains, and that they would focus their attention on killing him. His response was "bring it on!"
By talking to your player out of game about your plans, with as much or as little detail as you're comfortable with, you can gauge their reaction to your idea. If they're totally, completely opposed, you should probably change your plans, since your goal as a DM is to ensure the game is fun for all your players. However, my experience has been that, given enough information to feel that they're "in on it", players will generally treat such plans as a challenge to overcome or as a story component to expand upon and play with.
Players like to have choices, and like to feel that they can influence the plot and change their own destiny. Offer them this opportunity, and they will rarely be upset.