Spiteful Behavior

Question
D-Lee,
Is it possible for labs/pups to be spiteful?  The reason for my question is that I have a 5 month old male lab pup.  There have been two occasions recently that have caused me to question this, both involve his liter mate who is my sister-in-laws dog, also male.  Once occurrence, Belmont(my pup) was being mounted constantly by Cooper (his liter mate).  Later that day, both dogs were laying down and Belmont walked over to him, sniffed him and then lifted his leg and peed on him!  (I am not kidding!!)  Most recently, after being bullied by Cooper all day long, Belmont walked over and peed on Cooper's bed!  
With these two exceptions, Belmont has not had any accidents.  
Is it possible that this was his way of retaliating?  
I have had a labs in the past that have exhibited similar traits!

Answer
Hello...both male dogs are Not neutered right? Males will mark anything if they are not neutered including other dogs my jack russell terrier male used to do this i couldnt have him peeing on the couch/chairs/my other dog! so got him neutered. This isnt a way of being spitful your dog just wants to show the other he is boss since mounting is a way to establish dominance amoung dogs by marking his territory Belmont is saying I am the boss and thats that! Cooper is trying to establish he is higher up in pack rank with Belmont