The doctor said "now let's all relax ok... We didn't bring you here." 
Slade saw the Scottish man was telling the truth. His eyes showed seriousness. It also hid a lot of experience, pain and loss. He put his guns away and said "alright so tell me who are you?"
"I'm the Doctor and this is Kirby and the guy who I in my TARDIS is Alexander."

The TARDIS no weapons system but a great defense system.