Thank you in advance for your responses.
You are required to appear if summoned and tell the truth about what you know. You might even be doing them a favor as your testimony might come to show possible inconsistencies in the case against them.
Either way the law protects you... if you feel after testifying that your in danger call the police. after you leave court and the accused does anything its now under the Criminal code of Canada. Just as if you are summoned to appear in court your obligated to appear..