Shambuka's death is one of the debatable issue. I have some POV on it.
An interpolation?
'Uttara Khanda' is believed as later addition. But some scholars citing whole khanda couldn't be interpolation. I am also had no connection between Ram character and Shambuka death. I'm believing it as some later scholars self appraisal for their caste.
Still I've to accept that as some translations had that part.
If it was truth?
If it was truth, we must compare merits and demerits. As per Shamsepha Ramayana, in Ram's ruling There was no death to young people. So it's hard to me believe that incident.
At Tretha Yuga,'Varna' followed very strictly. Varna based on Act , not on birth.
So born as Suda killed for doing penance is unacceptable.
(IN MB Karn also mistreated for suta putra but his martial arts highly appreciated)
But BBrahmana doing penances for lok kalyan not for their Growth. Vishwamitra only taught Ram divine archery. He could've kill those asuras. But as a brahmana , he must control his sense. That's why he wanted Ram to help. Similar case was happened in Thandakarunya forest.
So Shambuka intention wasn't good as per Ramayana.Ram kept Sita away from him just because of His act mustn't affect people welfare. If Shambuka act justified, then many suda,vaishya would do penances. It would've bring Imbalance in Karma system (It always Mentioned Karma was the best way to attain Moksha). So Ram may wanted to avoid that. in fact Ram was best known for deep friendships without barrier of their castes and types (Vibeeshna is Rakshasha,Guha is tribal, Sugreeva is Vanara) so accusing Ram have prior to caste is unacceptable for me whether he killed Shambuka or not.
Not only in Ramayana, in MB also 'Varna ' mentioned. In BG ,Arjun berates intercaste marriages (here intercaste based on Varna). He believed intercaste marriage will bring imbalance to karma system.
In today world, We are doing love irrespective of our Caste. Arjun said
What belongs Dwapara Yuga.we can't judge based on modern world. Ashwamedha Yaga horse was sacrificed for good, I'm following Ramayana but I too don't believe sacrificing Horse will bring good(definitely it isn't about Veg vs Non veg.).
Why should we go long? In 1950's Bal Vivah was so common. Today we made some rules. We are respecting few leaders who done BV. So Ram act on Shambuka may not right as per today scale. According to Ramayana, Ram did all his acts as per his morals and principles (Including marriage).some of them are unacceptable today. Some of them may be wrong. We must leave those mistakes and try to learn his good morals. Dictating him on his mistakes won't be good choice.