https://www.youtube.com/watch?v=oOYGG0sLpbw
Playing devil's advocate here ... in Boom and in the movie it's just a combat strategy where Shadow views Tails as the "support" member of Team Sonic. Because Tails possesses a high IQ, flight capabilities, and creates gadgets, he is an essential asset. Shadow takes him out first to neutralize the biggest strategic threat. In Sonic X, to be fair Tails tried to attack him first. Even if it's to protect Cosmo, an attack is still an attack.
