I understand the risk of mirror life i.e. mitochondrion that eat mirror sugar molecules.

But I think that mirror chloroplast, that make mirror (beta) sugar inside regular planet cells can be a good way to fix carbon, without any way to free the co2 back besides fire.

What's the danger in that?

Mirror chloroplasts do not seem very risky, but the ability to create them is probably close to equivalent to the ability to create whole mirror bacteria/algae, so figuring out how to make mirror chloroplasts is a very risky line of research best refrained from.

The danger is that (to quote Jurassic Park) "life always finds a way", and the risks are massive.

A good example of the risks of mirror chirality is thalidomide - thalidomide has two enantiomers (one that provides sedative and anti-nausea effects and another that causes the infamous thalidomide birth defects) which the human body naturally flip-flops when administered.

The risk to reward ratio is negatively misaligned (edited) when factoring the catastrophic risks and how easy it would be to weaponize (either on purpose or inadvertently).