This hole lumiere arc made no sense and was rushed, i can understand why MC woud be sympathetic towars her but it make no sense why Knight boy woud not only forgiver her but also blame himself for her Straight up cheating!
Here is why...
1° this society resembles 19h century's standards in noble relationships, in this time period just being in the same room alone whit another eligible noble man woud be considered cheating so Knight boy definitely woud feel cheated by her!
2° it makes even less sense if she didn't cheat on him and just had a light hearted conversation with princess fiancé. Did her father forbid her from even talking whit Knight boy? If just having someone to listen to her worries was all she needed then what the fuck was Knight boy doing? he said he loved her sooo much, so why didnt he talked to her about it???? And i feel like author also knows this a big plot hole, thats why they dont show any past interactions between Knight boy and lumiere ( all we gets is the "i loved you since childhood flashback") if they did, again Knight boy woud feel cheated by her!
3° even if whe ignore the plot hole, the assassination attempt is by itself confirmation of her cheating. She was banished from her family and the first person she goes too is the guy that basically caused it??? Why didn't she contacted Knight boy first? Is he that unreliable or unreachable? even if he mostly stays at the Knight barracks (were she coud stumble upon her father the Knights captain) it woud still be better than asking princess fiancé for help, and its not like she's a criminal on the run or banished from public places! beacuse she met whit him and hes basically royalty status, unless....
Yea i cant visualize any other reason to commit murder for someone unless you love that person, and even if you don't think so, still Knight boy woud probably be thinking "hum why didnt she reached me first?" "why woud she do something like that for him?" He woud definitely feel cheated!!
So im conclusion she belongs in the streets!!
Also she tried to kill a lord so execute her immediately!