I don't see 嘛 itself has any threatening mood, unless it's your sentence.
嘛 majorly represent the following moods:
1. As the way it should be, sure thing, no big deal:
e.g.
本来就是嘛!(It is the way it is! Should be in that way!)
有什么好怕的嘛?(What's so frightening? No big deal!)
去就去嘛! (So go, what's the big deal!)
2. Wow! so, not bad!
e.g.
可以嘛!(So you can really do it, not bad! I'm impressed!)
胆儿不小嘛!(So, looks you do have some guts!)
3. Pleaaaaaase!
e.g.
行不行嘛~~!(Is that ok?? Pleaaaase!!)
你就听我的嘛~~!(Just listen to me, pleaaaase!)
UNLESS,
What you heard is 干嘛 gan4ma2, which equals to 干什么, meaning "do what?" or "what (are you) doing?"