I hate to nitpick but technically libertarians (especially small l, but even big L, which are different, and you used small l), are fine with helping people and mutual aid and altruism. What they have problems with is that being compulsory. They think that if you want to go help say the homeless or single mothers or animals or whatever your prerogative is, you should willingly donate your money to the cause if you have the money to spare and the will to share, but you should not be able to use the government to point a gun at some other guy to force him donate to your pet cause (i.e their view of taxes.)

I mean, there are certainly things to be said about that as well, some people believe helping others should be compulsory for example, and some things become a lot harder to organize without taxes, but it's helpful to at least understand the actual argument of your opponent and argue against it coherently instead of pushing strawmen that make it seem as if you don't entirely grasp their argument. A better example would be "any time taxes help someone a libertarian dies" for instance in this case.