This isn't about excusing what was done - Shipman holds responsibility for his actions, after all, as we all do - but it surely is sensible to investigate what causes a person to do this. For example, we now know curing clinical depression isn't just a case of 'gettting on with it and stopping being so miserable', but a case of an imbalance in brain chemistry, best cured by drug therapy (not disregarding human assistance, though, of course). To get to the point, we label Shipman 'evil' at our peril. To do so, simply excuses us from investigating its causes and potentially stopping another doing the same. Moreover, it stops us looking at ourselves too deeply; it's possibly too much to think that we're just a single chemical reaction away from doing something similar. To be evil is seen as cause enough of itself.
Well, Shipman was certainly dangerous and possibly mad, but no less culpable for all that. As for whether his actions were taking an easy way out, without firm proof of how and why it happened, we simply cannot know, it's merely idle speculation.