ばか！バカ！　馬鹿ー月！

Hello!
I'm reading a doujinshi and I'm in trouble with this sentence:

俺も半分遊ぶつもりで来たのに！

I think is something like "Half of the reason why I'm here is to play", but I'm not sure.
Any suggestion?

Then, what kind of onomatopoeia is ビービー?

Thank you so much in advance!

As for the sound-effect, check out this site: http://www.lolikon.org/misc/soundfx.html

As for the meaning of the sentence, it translates like this:

俺も = Me too
半分 = Half (of something)
遊ぶ = Play (as a noun)
つもりで = With the intention of / With the plan of
来た = Came
のに = Unfortunately, in spite of

Taken together, in a plaintive voice: But I also came half intending to play...

@Rpapo
Thank you so much! Your help is really precious

If you want to be really Anglicized with your translation, consider this very non-literal version:

"But I'm only partly here for the fun!"

(I'm trying hard to resist suggesting "But I'm only partly here for the lulz!" )

Then, what kind of onomatopoeia is ビービー?

Thank you so much in advance!

What's the object or person making such a sound? And for what reason? I can't really answer appropriately without the right context, so a full paragraph or a summary of the scene may help.

@ Kinny Riddle
Thank you!
Concerning the ビービー、could be a surveillance camera, I'm not sure. Maybe it's a girl who screams, like a kind of hysterical scream. It's not clear from the doujinshi page...I think I'll choose the beep-beep of the surveillance camera