Roland: I would suggest that you might be associating the phrase "moral responsibility" with more baggage (which it admittedly carries) than you need to. I find I can discard the baggage without discarding the phrase. That we call behavior caused by, for example, power lust, "worse" than behavior caused by a tumor, is like a convention. It may not be strictly rational, but it is based on a distinction. Perhaps it is more practical. Perhaps there are other reasons.

Imagine two cars, one of which is 1/5 as efficient as the other. We can call the less efficient one "worse", because we have defined inefficiency as bad relative to our interests. We do not require that the car have freedom of choice about its efficiency before we pass judgement. Many mistakes in philosophy sprout from the strong intuitive wish to endow humans with extra non-physical ingredients that very complicated machines would not have.