As much as I hate violence, I think that extreme cases warrant extreme measures. Belgium "easy" laws have good intentions, but I believe it failed to protect its citizens in the case of Patric Lumumba. Human Rights people will disagree with me, but I am all for the death penalty if it will save an innocent life. So yes, I think Belgium was legally guilty.
The Belgian government was legally guilty for not stopping the murder of Patrice Lumumba. They said so in 2002 when the Belgian government apologised to the Congolese people, and admitted to a "moral responsibility" and "an irrefutable portion of responsibility in the events that led to the death of Lumumba"
Once Belgium had been made aware of the threats against Patrice Lumumba's life, as has been reported, it had a legal, not to mention moral responsibility to at least make Lumumba aware of these threats. Belgium did nothing to act on these threats, violating the country's own 'Good Samaritan' law.
Was Belgium legally culpable for failing to prevent the assassination of Patrice Lumumba from taking place due to its own 'Good Samaritan' laws? I would argue, no. While being a good samaritan means keeping an eye out for others, it does not mean that it is their responsibility to constantly do so.