Looking back, Zabuza was probably suicidal already when he met Kakashi. Like, even if we don't look at the power creep later in the series, there's no reason to fight against Team 7 one at a time, or feed moves to them (ie. letting random(?) genin break his hold on Kakashi). He "killed his entire class", "assassinated the mizukage", etc. he's clearly feeling hopeless, depressed. It's really kind of sad.
