Could someone ever be considered significantly responsible for another's suicide? I don't mean to include cases in which, e.g., someone gives a weapon to an unstable person. The person I have in mind causes severe emotional distress to another person who ultimately kills herself.

Yes, though I wouldn't want to have to adjudicate responsibility in a particular case. Here's the philosophical principle I've got in mind: If a person A provides sufficient motivation for person B to commit an act, then A might be responsible for B's act. If A intended to provide sufficient motivation, intending that B commit the act, then I can't imagine A not being responsible. And if B would not have committed the act but for A's motivating actions -- in other words, if whatever A did was also necessary to B's committing the act -- and A knew this, then A would definitely be responsible. The problem with applying this general principle to suicides is that what counted as sufficient and necessary motivational conditions for a particular suicide are almost never known for certain. Most suicide victims (as I understand it) are assumed not to be in full rational control of their actions; e.g., there is mental illness involved. If suicide is an irrational act, then assigning...