OK, Having the disease MS does not make people mean. However depending on the form of disability that one has can change their attitude however they are in charge of this. And it can be fustrating when a family member says 'Oh, but you look healthy and you were able to walk 10 minutes ago or had energy 2 hours ago, etc...
I know this as I have experienced this and it did not make me an angry person. It only inspired me to educate these people of what MS truly is as it is an unpredictable disease of the Central Nervous System with the key word 'unpreditable'. Some finally understood and the ones who did not nor wanted to...well that is their loss. I know what I know and how to deal with it.
Also, medication can make one angry. When I would get a little snappy in a response my husband would ask "Did you take tabs (hydrocodone) today?"...my response would be yes and one of the side effects can be anger. I take very little now and if I find myself beginning to cop an attitude...I stop and rethink my answer.
I believe when a person knows what to expect from a disease and knows how to handle it as well as any medication they are taking and if / when the battle begins, they will be well armed.
Second thought, depending on where the lesions would be may actually affect cognitive issues and may alter a person's attitude but if the attitude is just towards one person than I would say that this is not the problem and cognitive / anger issue with an MSer should be managable.
~Zig