m'assassinent comes from the verb assassiner (to assassinate). This word is incredibly dramatic and attention-grabbing, especially for a pop song!
MIKA sings, "Tous ces regards qui m'assassinent" (All these looks that assassinate me). He's using this powerful word metaphorically to describe the intense, critical stares he feels from others. The looks are so harsh they feel like they could kill him, perfectly capturing a feeling of extreme social anxiety.
Have you ever felt like you just don't fit in your own skin? In this song, MIKA sings about feeling totally out of place. He says he's "Parfois beaucoup trop grand / Parfois beaucoup trop petit" (Sometimes much too big / Sometimes much too small) and that he lives by "en m'effaçant" (erasing myself). He wants to be "Libre, libre, libre comme l'air" (Free, free, free as the air), but admits, "Je sais pas faire moi" (I don't know how to do that).
This feeling comes to a head in the chorus. He's not just uncomfortable, he's "pas à l'aise dans mes blue jeans" (not comfortable in my blue jeans). He dreams of a passionate, fearless love like the one Jane Birkin famously had. Instead, he feels trapped by "Tous ces regards qui m'assassinent" (All these looks that assassinate me) and is so self-conscious that he "ose pas sortir de la piscine" (doesn't dare get out of the pool). It's a perfect image for feeling exposed and judged, wishing you could just tell yourself, "j'm'en fous" (I don't care).