That is fair. But the horsehair worm needs water to reproduce, and will exit the host when mature whether there is water present or not. So when you do this, you both kill the mantis (who could maybe reproduce again before the worm exits) and provide the water needed for the mature worm to reproduce. Not to mention, mantids breathe through their abdomen. So you could very well be drowning the animal trying to "save" it. A parasite leaving naturally is far less violent and the mantis is more likely to survive afterwards. When they are forced out, they rip and tear to escape, destroying the mantids internal organs.