At times like these, a temptation grips those who want to comment on recent events. A sentiment. An urge to proclaim, in solidarity will all of humanity, that all lives matter.
If you feel this, in any way, shape or form: don't. Resist. Abstain.
It's not a wrong sentiment to have, but it's a wrong sentiment to express. It does not help. At best, it adds nothing to the discussion, and only wastes the time it takes to express it. At worst, it causes all kinds of additional harm and strife, which is the opposite of what you wanted to achieve by expressing it. Expressing it will not improve the situation.
The reason for this comes down to one word: bandwidth.
Social situations have a very limited amount of bandwidth. Only a few things can be socially processed at once. In the cool, calm and collected moments between situations, things can be compared and contrasted and analyzed though larger perspectives. But when the situation unfolds, only a few key items that can be processed at a time. The fewer, the faster and better.
Saying "all lives matters" in a situation where some lives clearly matter less than others introduces a whole range of issues that cannot be processed there and then. There simply isn't enough bandwidth. Insisting that some of this limited resource is diverged is both counterproductive and - dare I say it - rude.
There is a time and a place for all things. Sometimes, tact and discretion are in order.