To say that someone has a 'bad temper' means that they are prone to getting very angry and can be quite aggressive and frightening when they are like this.
He is normally very nice but it is frightening to see him mad.
He's normally very kind but it's alarming to see him mad.
The two sentences you see provided above are excellent ways to express to your listener that a certain person is very nice but it is very scary to see that person mad. In the first sentence you will see the word frightening and in the second sentence you will see the word alarming. Both of these words mean scary.
He has a Jekyll and Hyde character that scares me to death sometimes
A Jekyll and Hyde character is one that can change quickly from good to evil (from the book by R L Stevenson).
It really depends exactly how the relationship is going at the present time. How critical do you wish to be about your husband?
EG "My husband goes crazy sometimes and petrifies me! Can I stay at your place?"
ジキルとハイドのキャラクターは、良い人物から悪人に素早く変わることができます。（R L Stevenson による著書）
どう表現するかは、正に現状における人間関係がどのようなものかによります。あなたは旦那さんについて、どれだけ批判的になりたいでしょうか。
例）"My husband goes crazy sometimes and petrifies me! Can I stay at your place?"
「夫がときどきおかしくなって、私は怖さで身がすくんで動けなくなるの！あなたのところに泊まってもいい？」
My husband is usually very kind, so it's a bit frightening when he does get angry.
I get a bit scared when my husband is angry, but only because he's so incredibly kind the rest of the time.
My husband is usually very kind, so it's a bit frightening when he does get angry.
Explanation: This sentence establishes that your husband is a nice person from the beginning. It makes the reason you get scared when your husband gets angry less about your husband's anger, and more about the contrast it is to his behavior the rest of the time. This sentence implies you don't get scared because of him; you get scared because it is unusual and you are not prepared for it.
I get a bit scared when my husband is angry, but only because he's so incredibly kind the rest of the time.
Explanation: By saying "he's kind the rest of the time," it shows your husband is normally a nice person, so makes your fear when he is angry less concerning. By saying "so incredibly" you are showing he is very, very kind, almost unbelievably so.
My husband is usually very kind, so it's a bit frightening when he does get angry.
（私の夫は普段はとても優しいので、怒ると少し怖いです）
解説：
この文ではまず始めに夫が優しい人であると伝えています。これが夫が怒ったときに怖いと感じる理由です。夫が怖いのではなく、夫が怒ることはめったになく、心の準備ができていないので恐怖を感じるというニュアンスです。
I get a bit scared when my husband is angry, but only because he's so incredibly kind the rest of the time.
（夫が怒ると少し怖いですが、それは彼が普段すごく優しい人だからです）
解説：
"he's kind the rest of the time" と言って、普段は優しい人であると伝えています。これによって、怖いと感じることと夫が怒ることの関連が薄くなります。"so incredibly" と言って、ものすごく優しいだと伝えています。
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