死党 has two main uses in modern Chinese. The first (and most common in everyday speech) means 'best friend / close buddy' — the person you're always hanging out with, like a 铁哥们 or 闺蜜, with no negative connotation. The second use, closer to the literal meaning 'diehard faction', refers to a fervent follower or a clique of loyalists, often implying unquestioning loyalty or extreme partisanship, and can be slightly derogatory. Which reading is intended depends on context: in a sentence about friends, it's the first; in political or fan culture contexts, it's the second.