Tom Strickler was a self-proclaimed "agent" for spies, helping them make contacts in exchange for money and favors. Michael Westen reluctantly approached him for help getting back into the Central Intelligence Agency, but their alliance was terminated when he learned that Strickler had helped Irish terrorist Thomas O'Neill find Fiona Glenanne, with whom he had a sizeable grudge.
Strickler claimed he'd done it for Michael's benefit - having an ex-IRA agent as a girlfriend was a liability. Michael saw it differently and shot Strickler. He then left a bomb that he was going to use to frame O'Neill at Strickler's place, thus implicating O'Neill for Strickler's death in addition to the 12 bombings in Europe for which O'Neill was already being investigated. This had the unfortunate side effect of causing an unidentified agency to freak out, which resulted in the death of Westen's CIA contact, Diego Garza, leaving Michael completely frustrated and with no clues as to who had killed Diego.