Picture a short, thick snake lying very still under dry leaves, close enough to touch and impossible to see. It does not chase anything. It waits, and the harm comes from how near it already is. Both uses of viper live on that one picture: first the animal itself, then a person who works the same way - close by, quiet, and dangerous exactly because nobody sees them coming.
From the word’s history
The name comes from Latin vipera, shortened from vivipara: vivus (alive) plus parere (to bear). Unlike other snakes people knew, this one was seen to bring out live young instead of laying eggs, and the name records that.
這個字來自拉丁文 vīpera，是 vīvipara 縮短來的：vīvus（活的）加 parere（生下）。古人看到這種蛇不下蛋，而是直接生出活的小蛇，就用這件事替牠命名。名字本身講的是繁殖方式，跟毒沒有關係；毒和陰險的聯想，是後來從牠的習性長出來的。
Wiktionary, "viper"
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