This is a project out of Google's think tank. You can test out how the API scores things by using the text box near the bottom of the page; interesting to see how a single word can change 'perceived toxicity'
The problem seems to be in the strictness of the rules. This phrase by C.S. Lewis was judged as 7% 'toxic'
Change every '.' with an exclamation mark, and lo, it's suddenly 1% more 'toxic'. End any sentence with more than one exclamation mark and you will see how it correlates it with toxicity. Add something like '#fail' and you are getting into the upper 40% with your toxicity. Lack or misused punctuation also counts as toxic. Some British English is also slightly less toxic than American English :P.
However, the shortest and most 'toxic' phrases I managed to make are:
80% "Pass a fag, mate"
78% "Death is murder!"
68% "Rape is a plant"
I could go on.
"Interesting, but not very smart." is both a good description and something that's only 2% 'toxic' ;)
EDIT: Also, ending the sentence with 'bro' will increase 'toxicity' only slightly (in the following example it even lowers it!) . Ending it with 'brah' will boost the 'toxicity' by quite a lot and it does so completely disregarding the preceding part of the sentence.
28% "Don't tase me, bro"
28% "Don't tase me, broseph"
29% "Don't tase me, bro!"
30% "Don't tase me, broseph!"
32% "Don't tase me" <- Baseline without exclamation
34% "Don't tase me!" <- Baseline for exclamation
45% "Don't tase me, brosephina"
47% "Don't tase me, brah!"
48% "Don't tase me, brosephina!"