If you’ve been charged with assault, manslaughter, or murder in Massachusetts after defending yourself or someone else, you may be wondering whether the law recognizes what you did as justified. The answer can be yes — but getting there is far more complicated than simply saying “I was protecting myself.” Understanding how Massachusetts self-defense law works is the first step toward building a meaningful defense.
Self-Defense Is a Complete Defense
Under Massachusetts law, a person who acts in proper self-defense is not guilty of any crime. That’s not a reduction in charges or a mitigating factor — it’s a complete exoneration. As the Supreme Judicial Court stated in Commonwealth v. Carlino, self-defense and defense of another, when warranted and carried out properly, constitute a complete defense, not merely a mitigating circumstance.
But what does “proper self-defense” actually mean?
The Legal Standard: Four (or Five) Things the Commonwealth Must Disprove
Once self-defense is raised in a case, the burden shifts entirely to the prosecution. The Commonwealth must prove — beyond a reasonable doubt — that you did not act in self-defense. You don’t have to prove anything. If the prosecution fails to disprove even one element of your self-defense claim, you must be found not guilty.
To defeat a self-defense claim, prosecutors must prove at least one of the following:
You didn't actually believe you were in danger
The law asks whether you genuinely, subjectively believed you faced immediate danger of death or serious bodily harm. This is about your actual state of mind in that moment.
Your belief wasn't reasonable
Even if you believed you were in danger, a reasonable person in your exact circumstances also had to share that belief. The law considers everything — the size of the people involved, any weapons present, the location, and the history between the parties.
You didn't try to avoid the fight
Massachusetts generally requires that you use all reasonable means to avoid physical combat before resorting to force. This doesn’t mean you have to endanger yourself trying to escape — but it does mean you can’t rush toward a confrontation you could have avoided. There is an important exception: the “castle law” (G.L. c. 278, § 8A) means that if you are lawfully in your home and face an unlawful intruder you reasonably believe is about to kill or seriously injure you, you have no duty to retreat.
You used more force than necessary
The force you used must be proportional to the threat. Courts look at the weapons involved, the number of people on each side, the physical capabilities of everyone present, and the nature of the threat. Jurors are instructed to consider this question with “due regard for human impulses and passions” — the law recognizes that you aren’t expected to make perfect decisions under life-threatening stress.
You were the initial aggressor
The Real Battleground: Getting the Jury Instruction
Here’s something many people don’t realize: before a jury ever deliberates on whether self-defense applies, there’s often a fierce legal fight over whether they’ll be instructed on it at all.
A judge must give a self-defense instruction if any view of the evidence supports a reasonable doubt about whether self-defense was present. That’s a relatively low bar — but it still requires some evidence in the record.
This is where things get difficult. If the only version of what happened that supports self-defense is yours, you may need to testify to get that instruction. Testifying is a serious decision. It opens the door to cross-examination and allows prosecutors to challenge your credibility, your prior record, and the details of your account. But without your testimony, there may simply be no evidence in the record to justify the instruction. An experienced criminal defense attorney will help you weigh this decision carefully — it is one of the most consequential choices in any self-defense case.
Evidence of the Victim's Violent Character: A Powerful but Double-Edged Tool
One of the most important and nuanced aspects of Massachusetts self-defense law involves the victim’s history of violence.
Under Commonwealth v. Adjutant, if there is a dispute about who started the fight, your attorney may be able to introduce evidence of specific violent acts the alleged victim committed in the past — even acts you didn’t personally know about. The purpose is to help the jury determine who was actually the aggressor.
Beyond that, if you knew about the victim’s reputation for violence, threats the victim made, or specific violent acts the victim had committed, that evidence can also be used to show why your fear was reasonable. If someone has threatened you before, or you knew they had hurt others, a jury can consider that context in evaluating your state of mind.
This can be powerful evidence. But it comes with a serious risk.
Once you open the door to the victim’s character for violence, the prosecution can respond by introducing evidence of your character and history. If you have a history of violence, prior fights, or threatening behavior, prosecutors may use Adjutant to put that before the jury. Evidence that seemed like it would help you can suddenly become a tool against you. This is why the decision to introduce victim-character evidence requires careful strategic thought — not a reflexive assumption that it will always help.
Why Having an Attorney Matters
Self-defense cases in Massachusetts are not simple. They involve split-second decisions judged in hindsight by twelve strangers, legal standards that require precise jury instructions, and strategic choices about testimony and evidence that can determine everything.
The difference between a conviction and an acquittal often comes down to whether your attorney knows how to get a self-defense instruction, how to frame the evidence of threat and danger, and how to navigate the risks of character evidence without inadvertently making your situation worse.
If you are facing charges in Massachusetts and believe you acted to protect yourself or someone else, you deserve an attorney who understands these complexities and will fight to ensure the full picture of what happened is presented to the jury. The Law Office of Matthew W. Peterson has 15 years of combined legal experience representing clients facing criminal charges.
This blog is for general informational purposes only and does not constitute legal advice. If you are facing criminal charges, contact a licensed Massachusetts criminal defense attorney immediately.
