N2
て当然だ
てとうぜんだ
Natural, As a matter of course
あんな生意気なやつは嫌われて当然だ。
It is only natural that a cocky person like that is hated.
Explanation
「て当然だ」 means 'it is only natural'. Given the reason in front, the result or reaction is just what you would expect. It can be good or bad: 「あれだけ練習したのだから、優勝して当然だ」 (After that much practice, of course he won); 「あんなひどい態度を取れば、嫌われて当然だ」 (With an attitude like that, no wonder people dislike him). Na-adjectives take 「で当然だ」.
Mixed up with
「〜て当然だ」: the reason comes right before it, so the sentence is compact: 「怒られて当然だ」 (He deserves the scolding).
「〜のも当然だ」: the whole event is the subject, so you can talk about someone else's reaction: 「彼が怒るのも当然だ」 (It's natural that he got angry).
「当たり前だ」 is more casual; 「当然だ」 is more formal.