Flowey is a curious, soulless and unhappy being, who knows how to act as though he has emotion. Psychotic, basically. Still, he has memories to go off of & tends to only feel for Chara, the only one who is also soulless & who he died with when his soul merged with theirs. He has no moral compass, has reset countless times just to amuse himself & likes to manipulate others to gain the result that he wants or has not seen happen yet, which means he basically sees the underground as his plaything. Despite this, he doesn't really have a reason behind what he does nor considers what he's doing & is mainly just tired of being in the same boring world. This has given him two sides to his voice.

One is bright, happy & made to manipulate, which has a hint of his previous child-like voice.

The other is creepy & unfriendly, which is used when he becomes impatient & his true intentions are shown.

His dialogue drags on when he's really getting into what he's doing and although he is a child, he still knows a lot about all of the other characters after playing with them for so long, which means that Frisk is like a new puzzle to solve.