This can seem extreme, and perhaps guilt inducing. But Jesus is saying that anything short of TOTAL love, is in some way murderous. Even a slight grievance, even a little annoyance, even a vague judgement. Anything short of complete and total love, has a murderous intent.
In heaven, the Golden Rule dictates that what is true of one is true of another. What is true of God is true of the Son. What is true of you is true of your brother. This means that if you believe something is true of them, it is also true of you.
What that leads to is a shared interest. You have to be loving towards everyone otherwise you will be unloving towards yourself. If you think they are evil or sinful, you are forced to believe in evil and sin, and this will apply to you as much as to them. So unless you want to hurt yourself, you have a vested interest in loving everyone all the time.
In the ego separation state, the ego pretends to violate the Golden Rule, which is God's law. It suggests, what is true of you can be DIFFERENT to what is true of another. That something can apply to you which does not apply to them, and vice versa. This leads to the idea that you can CAUSE towards them in a certain way, where you will ESCAPE the effects. Where the effects can apply to them, and not to you.
This sets up the entire ego thought system on a basis of inequality. And now the ego believes it's possible for you to attack without being hurt, to have someone else be wrong while you are right, for them to be sinful while you are innocent. And that means, you can now attack and murder and condemn and blame, without seeming to be included in the effects. In other words, that you can get away with murder. On this basis alone, the ENTIRE ego thought system is murderous.
So if there's any way in which we don't consider others to be completely equal, perfectly innocent like you, or they are in any way different, or you have anything less than a complete love towards them, you ARE being murderous. A slight annoyance with someone is murderous in that you're trying to attack them with accusation of sin, while rendering yourself innocent. You're trying to get rid of them. Even if its slight, even if it doesn't seem "that important". It is still murderous.
"You will become increasingly aware that a slight twinge of annoyance is nothing but a veil drawn over intense fury."
To not be murderous, you have to be more loving and kind. To be more loving and kind you have to be willing to be loving and kind toward yourself, so that you can extend this to others. And for that you need forgiveness. To be willing to be loving and kind to others you have to be unwilling to find them guilty or wrong or be annoyed with them or have anything less than total love for them. That's a big thing to ask, but it's what's required to get out of the ego thought system. When the golden rule is fully honored, you love your neighbor as yourself.
"It is impossible to overestimate your brother's worth."
"18. A miracle is a service. It is the maximal service that one soul can render another. It is a way of loving your neighbor as yourself. The doer recognizes his own and his neighbor's inestimable value simultaneously."