It is not explicitly stated what he has gained but...
"On the day appointed for the banquet, a grocer's van drove up from Willingdon and delivered a large wooden crate at the farmhouse.....and the word went round that from somewhere or other the pigs had acquired the money to buy themselves another case of whiskey." (end chapter 9)
Thus, it can be assumed that either he was traded the case of whiskey, or he was given the money with which he bought the whiskey.
He kills all who betray or plan to betray him.
Napoleon's motive for betraying boxer was to remove a potential threat. The animals rallied around boxer if his opinion would change against Napoleon he would loose some of his control of the farm.
he didnt
Betray them at the scene of the battle but is persuaded otherwise by squealer
Boxer is very important to Napoleon's success because no other animal on the farm works harder than Boxer. He is also the most dedicated and loyal worker.
because he wants to...
Boxer
He had been working too long and Napoleon thought he was useless to retire so early so he killed boxer
Boxer's tow mottos are: "Napoleon is always right!" and "I will work harder."
Boxer's two favorite sayings are:"Napoleon is always right.""I will work harder!"
He tells the beast that they must listen to Napoleon.
Napoleon sends Boxer to the slaughter house instead of the hospital because he doesn't want to spend money on something he doesn't think is needed, like treating Boxer. He figures that Boxer will never fully recover so he is useless to the farm, so he sends him away to the slaughter house to be killed. Hope this helped :)