I have heard some people utter "Marco Polo" in distress or shocking cases. Is it slang? Or is it used as something else? Can someone great as Marco Polo be used as an abusive word?
No, to my knowledge it is neither slang as such nor offensive in any way. One possibility is that Marco Polo is being used as a substitute for Motherfu..er. The rhythm is similar and Marco is close enough to Mother to give it a nice alliterative ring.
I have never heard it used this way and it is definitely neither offensive nor vulgar. However, if you have heard people use it as an expletive, they are probably using it because it is an alliteration of the other word. Think of it as an equivalent to words like darn or drat replacing damn.
This is an example of an unusual euphemism (warning: tvtropes).
Over time, euphemisms tend to pick up vulgar connotations of their own, a phenomenon called the euphemism treadmill. For example, toilet was once an innocent word meaning “a dressing room” (1819) but eventually became a euphemism for “lavatory or porcelain plumbing fixture” (1895). Now, toilet is itself a vulgar word, replaced in polite company with other euphemisms like restroom (1899) or powder room (1936).
To avoid the appearance of impropriety, people often use idiosyncratic euphemisms instead of established ones. That appears to be the case for Marco Polo! here, which does not turn up in Google searches for euphemisms or expletives. As others have noted, it has a similar cadence to Motherf—er! (or perhaps Jesus Christ!), much like the minced oath Cheese and Rice!
Because the whole point of an unusual euphemism is to use an entirely innocent word (which expresses vulgarity only by context), you needn't worry about people misunderstanding Marco Polo in other contexts.