I agree with Kimberley and VRbeauty. The only concrete explanation you have shared with us as to why you are staying is that your husband told you not to leave. This makes me concerned for both your husband's judgment and desire to look out for you, and your own willingness to look out for yourself. It's not about confrontation per se - your willingness to look out for yourself and not tolerate unacceptable treatment from others does not require anyone else's permission. For example, you write,
Is there a counselor or someone you can see to talk to in person? In particular, if staying really is the best option for you and your family, due to whatever logistical reasons, then you should actively make that decision yourself rather than just putting your husband in the decision making role and yourself in the martyr role. Then at least you will have made a choice to stay, however compromised a choice it was. A counselor might also give you some insight into more successful ways to interact with your MIL, or concerns about her behavior and the risks of staying that you might not have thought of. But it seems there is little we peanuts on the internet can do besides listen to you vent and offer our sympathies. And tell you that you deserve better! But you have to willing to stand up for yourself to get that better treatment sometimes. (And this is looking like one of those times when the better treatment is not going to be handed to you on a silver platter. You will have to demand it for yourself.)
But it seems you have given your MIL control over what you will or will not tolerate here. She reacts a certain way, so you decide not do speak up any more. You have abdicated to her childrearing decisions. You have given to your husband control over whether you will or will not leave an unhealthy living environment. This is your life, and you are posting here to complain about the admittedly horrible living situation, but you aren't owning your own decisions. By choosing to stay, you are tolerating all the horrible treatment. The cat-killing, and whatnot. Exposing your kids to the toxic treatment you are receiving from their grandma. Etc. It isn't a matter of speaking up to your not-so-rational MIL (trying to reason with irrational people is not usually a good plan), it is a matter of taking concrete action to get yourself out of harm's way.Like you said about "that it's more important to keep the peace than to speak honestly about how you feel or how another person's actions might be hurting or harming you," is NOT how I feel, however speaking out has only made worse. I'm a VERY confrontational woman. The deal is my MIL is not. She is bubbily while actually fuming with hatred. Jekle and Hyde. If I say one thing in defense, she flips out!
Is there a counselor or someone you can see to talk to in person? In particular, if staying really is the best option for you and your family, due to whatever logistical reasons, then you should actively make that decision yourself rather than just putting your husband in the decision making role and yourself in the martyr role. Then at least you will have made a choice to stay, however compromised a choice it was. A counselor might also give you some insight into more successful ways to interact with your MIL, or concerns about her behavior and the risks of staying that you might not have thought of. But it seems there is little we peanuts on the internet can do besides listen to you vent and offer our sympathies. And tell you that you deserve better! But you have to willing to stand up for yourself to get that better treatment sometimes. (And this is looking like one of those times when the better treatment is not going to be handed to you on a silver platter. You will have to demand it for yourself.)