If it depends on the personality of the character, what role is the mechanical threat playing?In 4e this never arises when I play a paladin, so for me a character who is struggling with the tenets of paladinhood in 4e... has no reason, IMO, not to take a ride on the dark side when necessary or convenient since there are no consequences.
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In other words it depends on the personality of the character.
In the real world some people make hard choices without the sort of immediate threat hanging over them that you are advocating for the paladin. They do it because they are committed. That commitment provides a reason.
A paladin is committed to honour, virtue etc. Those things are valuable in and of themselves, they are not simply means to power. Because they are valuable in and of themselves, the paladin sticks to them even when it is hard to do so. (And for the player, of course, it isn't hard because all the player is doing is choosing between two action declarations, either of which is a permissible move in the game.)