Maybe Alpert got pissed about Locke picking the knife because it meant he had to go back through everything again and make sure that Locke got sawyer to kill Locke's dad....

My explanation...



Ben killed his dad, and the island still made Ben sick eventually.

So when Locke picked the knife, Alpert had to go through everything again (since you cant travel forward based on some theories), and instead of Ben making Locke kill his dad, Alpert had to make sure someone else killed Locke's dad so the island wouldn't be pissed at Locke in the way it got pissed at Ben (hence the tumor).


Maybe?