As a child, Bruce Wayne became an orphan after his parents were killed in a Gotham alleyway. This began his journey of study and intense training to one day become The Batman, a Dark Knight seeking justice for his city. While the Caped Crusader has several methods of stopping crime, he only has one rule he refuses to break: He will not seek lethal justice, stopping short of killing the foes he goes after, not wanting to stoop to their level. It's a very noble, admirable, and honorable rule. However, it appears as though his faithful butler Alfred has an entirely different perspective. When Booster Gold breaks into the Batcave and is met by Alfred, the time traveling superhero has a good reason to fear for his life.
Alfred has been the butler of Wayne Manor for longer than Bruce Wayne has been born. He's known Bruce his entire life, and took care of him after his parents died, essentially becoming the closest thing Bruce has to a father figure. When Bruce returned to Gotham after a few years abroad training, it was Alfred who guided him and supported him, helping him craft the Batman identity into what it is today (as well as what it will be in the future). He was the Batman's first ally, and arguably most important, consistently pulling Batman into the light when he dives too deep into the darkness and despair. So when the sanctity of Batman's base of operations is broken into by an unknown assailant, Alfred doesn't hesitate to defend it and Bruce's identity, eliminating the threat by any means necessary.
As it turns out, the intruders are none other than the time traveling superheroes Booster Gold and his twin sister Goldstar as seen in 2007's Booster Gold #12. Due to his accidental creation of a time anomaly in the future, Booster is trying to borrow one of Batman's suits so he can temporarily pose as the Dark Knight in a museum robbery to reset events as they should be, eliminating any ripples in the time stream he might have created. To do this, they traveled to the past to a time before Batman and Alfred knew who they were, which is why Alfred is prepared to shoot them with a shotgun. As he tells Booster and his sister: unlike Batman, he's got no problem using lethal force.
The fact that Alfred is willing to do whatever is necessary, including killing, to protect Batman is equally awesome as it is concerning. Does Batman have a problem with Alfred's perspective? Does Alfred wish that Batman would take a more lethal approach to his crime fighting? The best guess? Probably not. Alfred most likely understands the reasons for Batman's one rule due to his past and probably wouldn't want it any other way. Also, as Alfred himself isn't going out in a bat suit to deal with criminals on a nightly basis, and seeing as he's not as capable and/or able to defend himself on the level Batman can, Bruce most likely respects and prefers that Alfred do whatever's necessary to protect himself and their secrets, as the scenario most likely doesn't happen very often. It'd be a problem if the Batcave were broken into every day. In any case, the knowledge that Alfred is willing to do what Batman won't is pretty interesting to think about.