[Morsul the Dark]

Well personally I feel Old Man Willow and Tom were at one point the same creature, that is not the discussion.

I think Not only did Old Man Willow hate those who walked on two legs, there are two reasons he would take on Merry and Pippin:

1: These horrid two-legged creatures Dared to Sleep against him using him like some common pillow. Willow doesn't like being used in such a manner.

2: Depending on his age, He could remember the old battle between Hobbits and the ld Forest. The great burning in the middle of it. Revenge is oh so sweet.

Ok there Is a third less likely option
3:Old Man Willow, somehow knew they were on an important quest. He felt they were being too aloof about the entire thing and needed a little bit of teaching about be cautious. I mean he didn't seem to want to Kill them. just punish them a little.