There's no way to make firing fun or even easy. When I had managers working for me, I wanted them to be able to fire, but always agonize about it. You're messing with people's lives here and there should be discomfort.
The key to effective, defensible, and minimum-pain firing is supervision. That's because the supervisor is the person with the most impact on both morale and productivity and because the supervisor is the person who is responsible for documentation.
The supervisor must set clear and reasonable expectations, and then check constantly for understanding. The supervisor must give frequent and usable feedback so course corrections can be made. The supervisor should be fair and consistent.
The supervisor must also counsel and correct. The supervisor must determine if poor behavior or performance needs to be documented and then document accurately, fairly, and rigorously. This usually means that there is a series of conversations between supervisor and subordinate where it becomes obvious that problems are being fixed or not.
If the supervisor does those things, then the person being fired may be sad, but he or she is unlikely to be angry. In smaller organizations this may not happen for two important reasons.
The person who is the supervisor may not know how to do the supervisory parts of the job. Few people get training like what I cover in Performance Talk or discover a book like it. Few have mentors who help them learn about supervisory skills.
The other reason is that in smaller businesses the urgent crisis of the moment often drives out the important but mundane work of supervision. In a small retail store, for example the manager (often the owner) may do ordering, merchandising, stocking and selling in addition to supervision. In a small manufacturing firm, the boss may also be the chief salesperson and supervise the shop floor and the office.
In smaller businesses what often happens is that supervision is seen as less important than the other jobs the manager must do and, usually, one that can be put off. To make matters worse, without training most of us are surpassingly uncomfortable talking to others about behavior or performance. And to make matters worse than that, management books are often vague and theoretical, telling managers to "motivate employees" or "be fair" without telling them how.
In smaller businesses, fair and minimum-discomfort firing can only happen if supervision is good. That means that the business managers need to see supervision work for the important and urgent work that it is and learn how to do it well.