Why

I have an 4 yr old male Lab
who is uncut
but the only why he is uncut is
because I want to bred him…
Anyways last sunday I bought a female
black lab puppy to bred with later on in the year…
Well he killed her
and I have no idea why he did that…..
Because he doesn’t act aggravting towards me and my family
or the other dogs we have….
if anyone has any information on this topic please email me at my email address which lilylostbaby@yahoo.com
please anyone I need help to understand my baby…..