“अहिसा परमो धर्म: धर्म हिंसा थेव च:”
अर्थात, अहिसा परम धर्म है पर धर्म के रक्षण के लिए हिंसा करना सर्वोतम है
That is to say, non-violence and not-killing is the highest dharma (do’s and don’ts), but fighting (including killing) to protect dharma is the highest form of living. So one can imagine how important this it. One needs to fight even to the point of killing in order to protect dharma including the highest dharma, namely, ahimsa or non-violence.
And the second verse to this shloka is:
धर्म एव हतो हंति धर्मो रक्षति रक्षित:
अर्थात, धर्म उसकी रक्षा करता है जो धर्म की रक्षा करता है, धर्म बिना का जीवन मृतु/जानवर/असुर/मलीच सामान है जो आप की ही मृतु करता है
This means, the person who spends his life protecting dharma, is protected by dharma back in turn; whilst the person who kills dharma is killed by dharma.
Hardik Bhatt of India writes further to me that this shloka is part of a conversation between Bhisma and Yudhister before the Mahabaharatta war.