Guilt almost always stems from a sense of 'should'.
As a humanitarian I 'should' not wish for someone's death.
But as a realist I know that death means an end, and sometimes when we just want something to stop, that end can look mighty appealing.
Perhaps instead of lashing yourself for the 'should' you can change it to the is. "I feel this way." With no judgements on why you feel this way, just that you do, accepting that you feel what you feel is nothing to feel guilty about. We are not in charge of our emotions, they will be what they are. How we act on them is what is important.
You can feel the way you do with no cause for self criticism - now, if you were to intentionally kill your mother, that would be where the problem would lie.
Let the emotions be what they are, and accept them. The actions you take in respect to those emotions are the significant aspect.