It's not a big deal if the tree becomes hostile. You can kill him with Buad's tea (found in the basement the healer/alchemist in Verdistis), to get some XP and treasures.
By the way, his name isn't Nemris. Nemris is the man who started the cult whose followers are trying to get into Nericon's garden. He wrote "The Path of Life" or some such book that you have to read.
