While your not wrong that reasonably Lexa heard what was going on there still wasnt any real indication of that. It shouldnt be up to the viewer to have to headcanon Lexas death into something less cheap then what it came across as. One draft had Lexa actively jumping in front of Clarke to shield her which keeps Lexas…

The problem though nothing in how it was written suggested she had any idea what was going on. They apparently wrote drafts where Lexa did in fact try to save Clarke but decided they way they went was better.