I'm very tempted to see it that way. Maybe because he was American (er...Canadian, I guess?) and thus wealthy and privileged compared to those he was trying to proselytize. Arrogance is in the mix, no question about it. But there was also a sense in which he was in the grip of an ideology that told him he had not just the right but actually an obligation to 'spread the word'. I don't feel I can entirely blame him for that part of it. He had a combo of privilege and brain-washing.
People evangelise for all sorts of things and the same problem arises in different situations, even if this was a particularly extreme example, given how vast was the difference between him and the targets of his evangelism. And while it can be open to criticism, the compulsion to 'spread the word' is often driven by a strong sense of duty. It's something people feel they are obliged to do. Seems to me it's a continuum and it depends critically on how large is the social gulf between the proselytizer and those they would proselytize.
E.g. i think of relatively middle-class leftists I have known, going canvassing around council estates, trying to get the proles to be more revolutionary!
(it's why I don't really feel that hostile to the Jehovah's witnesses who keep calling round here - at least they actually believe in what they are saying, unlike the spivs and wide-boys who used to call to try to get me to switch utility suppliers...the latter I would happily shoot with an arrow)
The problem with excusing the proselytizing of unproven supernatural beliefs because "brain washing" is that humans evolved as sentient beings with the capacity for reason and critical thought. Not everyone who is exposed to bad ideas accepts them. There are plenty who are raised by religious parents who ultimately reject it and go their own way.
Another problem is that this sense of "duty" you refer to - which I agree is very real to them - gives them a sense of grand purpose. To literally save people's eternal souls. This puts them above concerns of humanistic morality and legal codes. It leads to such things as breaking laws, trespassing on private lands where you aren't wanted, and potentially exposing vulnerable people to deadly pathogens.
It also leads to something which is incredibly common among missionaries: dishonesty as to their true purpose.
My first experience with religious missionaries was at the age of about 11. Another boy I played with at school kept asking me to come to a weekly social gathering which he called "Thursday Night Live." This evidently was a play on the show "Saturday Night Live" which was then popular among young people. He said I would be hanging out with other kids my age, listening to "cool music" and that we'd get cookies and punch. That last one, the promise of sweets, was like catnip to my young ears.
Fortunately, I told a mutual friend that I was going to ask my parents if I could go. The friend told me not to, that he had gone to it a month before. He said, "dude, all they talk about is God and Jesus. The music isn't any ACDC or Led Zeppelin (this is what the "cool kids" were supposed to listen to in those days). It was all about Jesus and stuff."
I then went back to the kid who had asked me to go, and asked why he had not told me it was a religious thing. I'll never forget what he said. "My pastor asked me to go around school and ask all my friends to come. He said not to mention anything about religion because then their parents might not let them go." This pastor intended to deceive not only me, but my parents, in order to lure me there with promises of sweets so that they could convince an impressionable young mind to accept their magical beliefs. This man was a liar who employed children to implement his deceit.
This experience has informed my view of religious missionaries. Unfortunately it is not an isolated case. Missionaries the world over lure people with kindness, only to later expose their true and only purpose. This guy Chau said in his diary that he intended to do exactly that. Get to know them for awhile, then eventually introduce them to "the Gospels." Yet that was really his true and sole purpose for going.
If we excuse not only people's magical thinking, but their immoral behavior toward others, because they were taught to believe certain things, well guess what? The people who taught them were also taught by someone else, and hence there will never be any responsibility fixed to anyone for this behavior. Sorry, but Chau was 31 years old, an adult who was responsible not only for the magical things he chose to believe in, but for the terrible ways in which he chose to spread them.