Bandit King manga is a bit tricky to pin down for a legit free read online. The title itself has variations in translation, and the official English release under Yen Press for 'Bandit Kings' is pretty new. So, scouring the usual suspects like MangaPlus or Shonen Jump's app won't turn it up. Your real options are either libraries with digital services like Hoopla, which sometimes have Yen Press titles, or using a free trial for a subscription service like Kindle Unlimited if it's listed there. I got stuck waiting for my library hold and ended up reading some fan scans from aggregator sites, but the quality was super inconsistent—one chapter had text bubbles totally out of order.
Honestly, the fan-translated route is chaotic. Some sites have it under the original title 'Tōzokuō', others as 'Bandit Kings.' The scans I found were from early volumes, and the translation got really rough after a few chapters, with confusing honorifics and weird slang. It made following the political intrigue around the main character, Harchen, a real chore. I'd say if you're patient, waiting for the official digital release to maybe hit a free preview or a library copy is the move. The art in the official version is so much crisper, especially in those big battle spreads.