Why do you think it's OK to summarily kill people for this kind of stuff?
Why do you say I think so? I do not, and I don't believe I've ever said anything to the contrary. I think one should only use deadly force to prevent death or great bodily harm to yourself or another person. I believe that under most circumstances killing or greatly wounding another person will be a horrible experience and should be avoided as much as one reasonably can avoid it, even when the law allows more latitude in your choice to employ deadly force. For example, while the castle doctrine may say that one has the presumption that deadly force is justified when defending against someone forcibly entering into one of the 3 places considered one's "castle," I personally wouldn't use deadly force against the intruder unless their intrusion also brought with it a reasonable fear of death or great bodily harm.
I think you're confused on a couple of things here. First you seem to think that there's no significant difference between what is a property crime (e.g., theft) and armed robbery (which is a property crime committed in combination with an assault against a person.)
There's a huge difference, and this is recognized under the law. If you left your cellphone on top of a restaurant table and someone snatched it as they walked by while you were distracted, that's a theft. If that same person held a knife up and said "give me your cell phone!" that's not only you losing your phone, but having your life threatened too: an assault. You seem to think "Well, the net result in either case is a loss of a cell phone. Therefore, there's no big difference in the crimes committed." You ought to know that under the law one CAN use a reasonable amount of force to protect property. But one cannot use deadly force (at least not in Wisconsin) to protect property alone because it's not considered a reasonable amount of force. Also while snatching your phone from the table would be considered a misdemeanor, taking even a penny from you at gunpoint would be a felony.
The other thing you're confused about is the Castle Doctrine. There are a couple of things at work here. In Wisconsin when a person claims they acted in self-defense, a prosecutor when deciding whether to prosecute that person, or a jury when deciding to convict or acquit, will look at a number questions. One of the questions they will examine is whether the person who claims self-defense used a "reasonable" amount of force (or threat of force.) In 1999 a state appeals court ruled that while there is "no duty to retreat" from a threat in Wisconsin, still it is permissible for a jury to take into account whether an opportunity to retreat was present when deciding whether an actor used a reasonable amount of force in their self-defense. If there's no reasonable means of escape, then it's not an issue. But if one fails to take advantage of an opportunity to flee when such an opportunity is present, the jury may decide that the actor used more force than necessary to prevent or stop an unlawful interference with their person. Since one can only use "a reasonable amount of force" that puts some self-defense claims on uncertain ground: on one hand the law says you have no duty to retreat, but on the other hand the courts are saying if you fail to take advantage of an opportunity to flee your self-defense claim may be damaged. The Castle Doctrine is based on the concept that if you're in your residence, your vehicle or your place of business, you already are in your place of retreat (a/k/a your castle.) The Castle Doctrine therefore removes, at least for those 3 places, the question of whether you have an opportunity to retreat out of consideration. Failure to retreat from where is already considered your place of retreat cannot be held against you, as it can be apparently everywhere else you may be under that 1999 case. The other thing that the Castle Doctrine did, and perhaps this is the part you object to, as I understand it, it basically says when you're in "your castle" you don't really have to make a determination of the intentions of someone who is in the act of forcibly entering your castle. You don't need to figure out if he or she is simply coming in to get your stamp collection, or if they are coming in to get you, or your child. The law now says if you're forcibly entering a home "it sucks to be you" because that homeowner or business owner doesn't have to standby to figure out why you're breaking in. Now I highly doubt that prior to the Castle Doctrine law many prosecutors in Wisconsin actually attempted to prosecute a homeowner for using deadly force against someone breaking into their house. Most people would probably be outraged at a prosecutor for trying such a thing. People want to feel that they can be secure in their homes and that the law will grant them considerable lee-way and support in how they protect themselves there. So I am guessing that the part of the Castle Doctrine that you find most objectionable is also the part that probably isn't really changing things much from how they were prior to its passage.