What would you do if the only stress you had in a relationship was their child?

My boyfriends son is a difficult child, young, around kindergarten, and threatening to kill people and hitting teachers, which caused cps to get involved. I'm concerned for him as to why he's doing this and concerned for my future with my Boyfriend, will his son affect my children? Is it his parenting style? Any advice?
What would you do if the only stress you had in a relationship was their child?
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