Abortion should be illegal.

One form of harm is potentially direct harm to the fetus as a morally relevant entity. The harm to the fetus is difficult to quantify, but "difficult to quantify" does not automatically mean "little enough harm that it is automatically outweighed by the benefit to the woman".

Abortion should be illegal.

The harm of abortion outweighs the benefit.

Abortion should be illegal.

Even if that were granted as true, which is not a given, that doesn't make abortion good or neutral. It merely moves the goalposts into the domain of balancing the harm of abortion against the benefit.

Abortion should be illegal.

From a utilitarian or consequentialist perspective, uncertainty is factored into moral calculations via multiplying probabilities. Take whatever your suspicion is that abortion might be as-bad-as murder (as a percentage) and multiply it by the number of abortions.

Abortion should be illegal.

There are roughly 700,000 abortions in the US per year, worldwide there are as many as 60million. If there is even a 1% chance that a fetus has human-level moral worth, OR that a fetus has 1% of an adult's moral worth, legal abortion allows 600,000 "morally-as-bad-as-murder" acts per year worldwide.

Abortion should be illegal.

The Biblical definition of murder is not particularly relevant.

Abortion should be illegal.

This is only true if you exclude fetuses from the definition of "society". If fetuses count as people, then abortion is the extermination/murder of a segment of society.