Kiruv has been a concept since the days of Avraham Avinu, and Hashem even had Noach build the teva for 130 years in order to give people the time to do teshuvah. Noach, despite being a tzadik in his days, did not go out and actively encourage them to do teshuvah. He should have.
It is wrong to put blame on people who are m’karev other Yidden. They are doing what we all should be doing on some level. To blame those in kiruv is to blame the Skulener Rebbe zt”l, Rav Meir Shushter zt”l, the Lubavitcher Rebbe zt”l, etc. One who is not on the level of a gadol should never open their mouth against one is.
I, for one, know that I would have benefitted tremendously from those who put themselves out there for others. Where would I be without them? My Torah, is their Torah. My family, is their zechus. I became frum because of those wonderful souls who shared their love of that which is Pure and Perfect.
The real answer to why there are missionaries sneaking into the frum community is that we are very clearly in the Messianic era, and the Sitra Achra is upping its battle, trying to prevent Yidden from cleaving to Hakadosh Barush Hu.
Missionaries have been developing apologetics and tactics to worm their way in. You can blame Artscroll just as much as you can blame those putting tefillin on people; the doors to our Torah became open to them. Is Artscroll wrong for helping those who want to learn able to learn in the language they know best? Of course not.
The missionaries who go undercover are just following the teachings of their religion, which instruct them to present themselves as a Jew to the Jew, and a gentile to the gentile. It’s evil and corrupt, and sheker v’chazir. Like a pig, they stretch forth their feet — in this case, a tallis and peyos — and claim kosher. But inside they worship a man-made man-god.
Don’t blame the kosher for the unkosher who worm their way in.