If taxation is done as intended, it definitely is not theft. However, it's only human to want to use taxpayers' money for personal financial gain and, probably, very few people can resist it 
What does it matter what a thief does with the money once he has stolen it?
There are two separate issues here. How the money is obtained. And how the money is spent.
How the money is spent does not affect how the money is obtained.
That is what people do not understand.
If I kill a guy, then roast him up in a fine meal that I then give to some hungry people on the street, would you be trying to justify the murder because some people got fed? Would you not even care that someone was killed? Would people even have to ask the question "is killing people to feed people murder?". Would you be criticizing those that call it murder as being heartless? Not wanting hungry people to be fed. What about those poor starving children that just want a meal? How dare you call it murder?
If this has been done for millennia, nobody would consider it murder. If someone else chimed in and said "maybe we can feed people without killing people" then the majority would have no clue how it could be done. So they would say that the killing is justified. Oh, but only "if the meal is prepared correctly". Because they cannot fathom any other way.