This is because we have to remember April is convinced that Becky is going to betray and destroy her because she's cuter and thus everyone loves her better. I can remember her feeling quite put out that people might actually adopt the cute name Becks assigned Mr B because that meant to her that everyone loved Becky and wanted her to just shut up and stop being a little brat about coming in last place to the pretty child.
Not, of course, that April is all too eager to have this tendency pointed out. She angrily denies that she's bitterly jealous of Becky because to have envy in her heart to the least degree would make her a depraved, heartless monster no one could love or sympathize with. Becky MUST have aggressive intent in order to justify April's hatred because the alternative means looking in the mirror and seeing a gloomy child who sits in her room and wonders why no one really loves her and why everyone wants her to be a third-tier extra in her own life.
If this is starting to sound familiar, let's remember also that April also sees Liz as brining a lot of her problems on herself by being an idiot. What this tells us is that April comes very close to being a female version of the Delicate Genius. The fit isn't quite exact because of a horrible reason but you can see that they have a lot in common.