The wizard died... but his familiar didn't.
Familiars are pretty smart. Even a low-level wizard's familiar has an Int of 6 or more, which is animal cunning combined with some simple reasoning. The master's death turned it mean and vengeful. It might not be able to kill its master's murderer, but it could sure torment him.
Physical torment is easy. The creature is probably very small, so it is easy to make off with the character's smaller, yet vital, supplies, such as healing potions. Depending on the familiar and the setting, it can be very difficult to track; which of the ravens in the town common stole the magic ring?
Psychological torment would be more difficult, but more interesting. The character sees the familiar everywhere he goes, staring at him, but it retreats when he draws near (into the wall, flies away, whatever). That's all it ever does, stare and retreat. But its always there. This results in fun when the PC starts tearing apart the inn walls screaming "THE RAT IS STARING AT ME AGAIN! AAH! THE RAT!"
In a long-running campaign, the familiar could become sort of an "anti-familiar" for the PC. The friendly bond it shared with its master has been severed by the PC, and it is now linked to him in an intimical fashion. It could gain new powers as the PC levels, just like it would have progressed with its former master. It gets tougher, smarter, and meaner, keeping track with the PC. Its presence could be disruptive the character, giving a -2 to Spot and Listen? Perhaps some kind of template is in order.
Note that none of these ideas would work very well with a toad.