Poor Molesley, not only did Bates steal his girl, his job as Lord G's valet, the love of his life (the shoehorn of course), and his job as Matthew's valet, now he's stealing his position in the Crawley House as well??? And was it Bates who infected him with the flu?

Molesley must have been so happy when he heard that his master was paralyzed. Finally, there'd be more to do for him than just watch him dress. He must have been so crushed when he found out that once again it's the lame Bates (who himself can't even walk without a cane) who will be pushing Matthew's wheelchair around, carrying him up the stairs and dressing him up.

If that's not a legit reason to murder this guy (or murder his wife and point the evidence at Bates), than I don't know what is.