The Wicked Willow of Withywindle

[Legate of Amon Lanc]

Quite an interesting question, Inzil. Well, as for me, it seems obvious that Old Man Willow's ultimate motive was that he "hated all that walked on two legs", as those evil trees do. But what exactly was the point with Merry and Pippin, well... not that the trees would be carnivorous, but when you make the soil fertile with some organic remains, it is always more nutritious... well, hm, I think you get the point. Disgusting.
Or for that matter, it could be simply trapping them. The malicious intent. That actually sounds even more plausible and "in character" for the Old Man Willow. For the tree, it does not matter how long he keeps them trapped, eventually, they will die of starvation or something. Enough for the evil tree to be content: two more of the terrible disgusting walking things destroyed. Why didn't he kill them straight away? He wanted to enjoy mocking them (and Frodo, who was totally helpless and could not do anything) and make them suffer for a while. That would quite fit. Once again, so lonely in the forest for such a long time, I doubt he had encountered Hobbits in a long time, he wanted to enjoy the moment.