This was a bad man. 這句說話有錯嗎?

As you are talking about the bad man, so the pronoun he should be used. If you use the verb 'was', it means he is no longer a bad man now. So you have to decide what meaning you want to convey. However, if you are giving a general comment about a person, the present tense should be used.

He was a bad man or He is a bad man.

This can be used as a determiner or a demonstrative pronoun. If it is used as a demonstrative pronoun, it normally refers to a dead object.

This is a very good sentence.

If you want to use it as a determiner, your sentence should be:
This man is bad.

This was a bad man. 這句說話有錯嗎?
應是:
He once was a bad man 或 He once was a bad guy. 因為第一句聽來好像:
He once was a Bat-man (蝙蝠俠)

如果你要說他是一個壞男人: He is a bad man
如果你要說他曾經是一個壞男人: He was a bad man
但其實壞男人是什麼???
如果是專騙女孩的: He is a scammer
如果是吸毒的: He is a junky
如果他是三合會人士: He is a triot member
etc....

He is a bad man! 我深信會好一點。

死物才會用 : This. "was"有點被動及過去的意思。佢係壞人就係壞人啦！

我Ｄ英文唔算好，但只知有一定的規則架！

如果你想話呢個人過去係 bad man ~ 而有前文提到既
應該都 ok 既 ~~
但係最好都係 This is a bad man.
或者 He is a bad man.

no wrong but ' it was a bad man' 會好d