、、。
Dammit, that bastard Nagisa really went and ratted me out to the teacher.
Literal
Dammit, Nagisa [gen-の] bastard-やつ, really teacher [to-に] snitched-[damn-it-やがった].
やつ after a person's name is a rough, dismissive 'that guy / that bastard' — showing strong negative feeling toward the referent. チクる is slang for 'to tell on, to rat out, to snitch' — originally from a mimetic (チクチク 'jabbing'). The auxiliary ~やがる attached to the て-form conveys strong contempt or disgust toward the action — 'damn it, they did X.' やがる is harsh masculine speech; never use in polite contexts. The whole line stacks multiple rough features for intense frustration.