Unfortunately I'm fairly sure you're not the only person who is thinking this way. The reality is that thousands of people come to Tobago and have a perfectly wonderful experience and the chances are so will you. I agree with a lot written in these threads and some i strongly disagree though i also agree that these recent incidents have been misrepresented by the international media. Of course they have a right to report on such matters but niether of these brutal incidents involved tourists. I think in general tourists dont have too much to worry about, no more than going anywhere else in the world. Partly it depends on where you stay, where you frequent and how you interact.
Google "Tourist Murdered" and you'll see In the last 12 months tourists have been murdered in Spain, Cuba, several in Thailand, Puerto Rico, several in Antigua, India, Dom Rep, I lost count of how many in Mexico and several in China even one at the Beijing Olympics, would you have cancelled your trip to any of those places? Or would you have cancelled your trip to the Olympics after the American was stabbed in the street? Have you ever seen the crime statistics for Miami? or Florida, which is still the number one destination for UK tourists. Six years ago in St Lucia i witnessed two tourists robbed on a popular beach in broad daylight by two men brandishing cutlasses, that was six years ago, (and by all accounts according to friends who live there it happens fairly often there), but how many people still go there on honeymoon?
The reality is that the crime figures for Tobago are very low, they are however collective with Trinidad so seem disproportionate for the size and population of the island, and what crime there is generally does not affect tourists, even these recent attacks were not tourist related. That's not to say things dont happen to tourists, that would be naive and stupid, thanks to cable and satellite TV we are now dealing with the Americanisation of Caribbean youth culture and it's negative effects, but these attacks were extreme and rare and people should understand it's not the norm, even so tourists should not allow this kind of thing to deter them from coming to what is fundamentally a fantastic place to holiday, because if you did, you'd never step out of your house and you'd end up living in a bubble, and with the best will in the world you could go online and research and find somewhere seemingly safe like Phuket, Goa or Cancun and still end up a statistic.
So no, dont cancel.