Someone did not want Anthony and Gust to form an alliance.
Two forces, probing each other, without a foundation of trust, just a little sabotage could create irreparable doubts and lead to the inability to cooperate ever again.
There was no need to do much; one only had to know where Anthony went to the Prison of Death, then rush over there, kill all the Undead Guards, and report back.
Gust would send people to investigate, and when they saw the Followers of Light unharmed while the Undead Guards lay dead, the first suspect would definitely be Anthony.
Negris could see at a glance that it was a frame-up because Anthony was with them the whole time. Gust wasn't there; they hadn't even seen each other's faces, lacking a foundation of trust.
They'd rather suspect that there was a traitor framing Anthony than believe that Anthony was the perpetrator.