Thanks for the welcome and the questions. In short: B doesn't. The purpose of property rights is to avoid the initiation of violence, therefore property title can't be legitimately transferred through the initiation of violence.

A few clarifying questions, though:

• Why is there an assumption that B wouldn't face any consequences for engaging in murder, trespass or theft?
• How would murdering and stealing be beneficial for B if B is faced with the prospect of being ostracized or having violence used against himself?
• How long is that going to last before people take notice and retaliate?
• Given his demonstrated preference for violence, what expectation would B have that no one would use violence against him? Wouldn't any use of force against B post-murder be retaliatory rather than initiatory?
• Is B just going to kill people every time he needs something? Wouldn't that create a huge risk of retaliation?
• If B is so dependent on the labor of others that he thinks he needs to kill someone and steal their house, is B's best choice really to declare war against everyone else?
• How would B continue to "mix his labor" with the stolen property for years? Wouldn't killing people make homesteading a little too risky? How could he plan for the future if there's always a fear of retaliation?
• If he was already capable of homesteading and mixing his labor with scarce resources, why did he need to kill anyone in the first place?

I'm not trying to be obtuse; I'm just having a hard time fathoming B's motive and the circumstances in which something like your lifeboat situation could actually happen. Thanks in advance!