Should I be worried?

My aunt’s child is honestly deranged . This boy has always talked about wanting to kill someone he is only six years old . Well today he grabbed my child’s neck and told him to look at him . My son is four and he smacked him and told me . My aunt is mad at me but sat right there and let her kid do this . My aunt is only 9 years older than me my grandma had her at 52 . We had a great relationship but she never disciplines her kid , he is seriously dangerous what should I do ? I suggested that she get him evaluated but she said no . What six year old wants to kill someone ? What six year old wants to choke & hit people ? I usually chalked it up to him having an active imagination but no he’s so serious . My cousin is 14 and she is terrified of her brother , he really needs help this is weird right ? he use to be such a sweet kid but now all he wants to talk about is killing things . I wish I was making this up but he freaks me out

UPDATE 1: after yesterday’s incident I told her he is no longer welcomed . My child will always be my top priority right . She’s angry at me but she’ll eventually get over it .

Update 2 : I’m calling cps tomorrow because this is extremely unhealthy , my mom said the most they can do is baker act him .