I respect that 'motherhood' doesn't come naturally to some. I, myself, do not know any 'perfect' parents.
I understand the struggles of motherhood as well. I only have 3 boys, ages 4, 3 & 3. They have all pushed me to my limits, but, NEVER have I thought of killing them. When I am feeling overly stressed, I reach for the phone, to just talk or ask someone to come over and give me a break.
When children are killed by their parents, a definite 'line' has been crossed. Do I think all parents are capable of killing their own child? I don't think so, but I am not 100% sure.
The fact is, everyone is responsible for their own actions, regardless of their circumstances. For all the excuses out there for killing your child, I can think of more excuses to not.
I applaud you for raising 6 children, I'm sure it was alot of work.