I don't believe that there is a red line at all.
Putin will continue to slaughter Ukrainian children for as long as he feels like it and, if the mood takes him, he will use chemical weapons knowing full well that NATO will do absolutely jack sh1t about it because Ukraine is not in NATO.
On the same basis, there appears to be nothing to stop Putin going on to invade Moldova, Finland, Sweden etc. etc.
Putin will continue to slaughter Ukrainian children for as long as he feels like it and, if the mood takes him, he will use chemical weapons knowing full well that NATO will do absolutely jack sh1t about it because Ukraine is not in NATO.
On the same basis, there appears to be nothing to stop Putin going on to invade Moldova, Finland, Sweden etc. etc.