Okay. If you've seen Lion King 2, do you know the Outlands? Yes, these changelings do not live in a paradise, as mentioned above. Okay. Ponies are scared of changelings, this must be sure. Even before they attacked, an insect pony will creep anypony out. Added with the fact that they eat love, Changelings would've been shunned as monsters in ancient and modern Equestria alike. Regarded as monsters, to extend. To live in isolation from the rest of society, it would be hard. No actual ponies would equal no actual food. It is unknown if love can be extracted from animals, so I'm going with the fact they need ponies to survive. This all leads to one point.
Changelings do not get much food. Now, if they were animalistic hive minds, the Queen would send the workers to find food. Even if she did, they must have come to know of Equestria. But we know changelings are sentient, so they must have looked AT the Queen to give them food. This is seen in Equestria too, as ponies ask the Princesses for help in times of need.
She had to feed herself and her subjects. The way she did it was indeed evil, but the fact remains that she HAD to.