、。
In short, he was just too honest.
Literal
Essentially, he [topic-は] too-much honest-too-much.
あまりにも is an intensified あまりに ('too much / excessively') — the も adds emphasis. Paired with the ~すぎる suffix in 正直すぎた = double emphasis on 'too much.' This stacking of intensifiers captures the dramatic 'just too [adjective]' nuance.