Be respectful and kind, but put your safety first. Plan your exit strategy carefully with that top of mind.
Relationships are hard, even when people are even-tempered. I am sorry you are in a relationship with someone who has a terrible temper, and it's good you realize you need to break it off. Be mindful of how valuable you and your time are, and make an effort to surround yourself with supportive, kind people.
Proper etiquette posits that you be aware of others' feelings, treating them with respect and empathy. Try to use these values when breaking up, but your safety is the most important consideration. So, if you have ever felt scared or unsure if your partner's awful temper could escalate, put your safety first, and etiquette second, to see you through the breakup.
Do some groundwork to get ready. Decide what is best to do regarding any of your partner's belongings that you may have. Next, decide on a location for the breakup. For example, should it be a public place, for coffee? Or, if an in-person breakup could be potentially harmful to you, consider a phone call or FaceTime. Only you know the best answer to "where," it should happen. Now, let's work on our speech.
Be aware of any subjects or words that trigger this person's terrible temper, and avoid them if possible. Be confident, secure, and look the person in the eyes whether you're in-person or on video chat. Say something like:
"Thank you for meeting me today. I have felt that we are not a good fit for some time, and I believe it is best for both of us if we break things off. I appreciate your understanding and respecting my decision. I wish you all the best."
Because I do not know this person, it is hard to give more specific advice. However, from an etiquette viewpoint, keep it respectful. Do not bring up old wounds, or give elaborate reasons. Keep it brief, simple and to the point.
Hopefully, you will hear an equally respectful acknowledgment of the breakup. If not, and tempers flare, say, "I'm sorry you feel that way," and remove yourself from the situation. If you have decided to break up in-person, consider asking a friend to wait for you, perhaps in the car or nearby, and then you can leave together. Regardless, be prepared to go, or say, "goodbye" straight away.
One of the myriad terrible consequences of the global pandemic is a rise in domestic violence. If you are in this situation, please seek professional help. Find a safe spot, if you need one, and call 911 or the National Domestic Violence Hotline at 1-800-799-7233, https://www.thehotline.org.
Stay smart and stay strong, and be as respectful as you can through this breakup, but put yourself and your safety first. Remember, you are worthy of being in a beautiful relationship, one that is nurturing, and full of love and appreciation. I wish you the best of good luck, and I hope you find peace.