OK,

What is the interrest to use a Map ?

I have modified my trigger but there always only 52% of code covered

trigger triggerAfterInsertLead on Lead (after insert ) {

    if(checkRecursive.runOnce())
    {
        Set<String> leadmail = new Set<String>();// liste des nouvelles pistes (par email)
        List<Lead> updatedLead = new List<Lead>();
        List<Account> updatedAccount = new List<Account>();
        List<Lead> deletedLead = new List<Lead>();
        String demande ='';
        boolean existingAccount = false;
        String URL = ' https://cs16.salesforce.com/';
        
        for (Lead l : trigger.new)// ajout des nouveaux id
        {
            leadmail .add(l.Adresse_mail__c);
            System.debug('tentative d\'insertion de la piste :'+l.Adresse_mail__c);
        }
        
        //récupération pistes et comptes existants

        List<Lead> leads = [SELECT  Id, Salutation, FirstName , Adresse_mail__c, LastName, Num_ro_piste__c, LeadSource,Phone, Telephone_portable__c, Rating,Type_d_utilisation__c , Origine_connaissance_Kiwatch__c, Client__c, D_sinscription_la_newsletter__c, Inscription_la_newsletter__c, Raison_de_la_d_sinscription__c, Date_de_d_sinscription_la_newsletter__c, Date_d_inscription_la_newsletter__c, Street, City, State, Country, PostalCode, accepte_jeux__c, Date_inscription_au_jeu__c, Email_parr__c, Email_parrainage_2__c, Email_parrainage_3__c, Email_parrainage_4__c, Email_parrainage_5__c  from Lead WHERE Adresse_mail__c IN :leadmail]; 
        List<Account>accounts = [SELECT Id , Salutation, FirstName , Adresse_mail__c, LastName, PersonLeadSource, Numero_client__c, D_sinscription_la_newsletter__c, Inscription_la_newsletter__c, Raison_de_la_d_sinscription__c, Date_de_d_sinscription_la_newsletter__c, Date_d_inscription_la_newsletter__c  FROM Account WHERE Adresse_mail__c IN :leadmail] ;
  
         Map<String,Lead> leadMap = new Map<String,Lead>();
         Map<String,Account> accountMap = new Map<String,Account>();

        // fill up map
        for(Lead lead :leads){
           leadMap.put(lead.Adresse_mail__c, lead);
        }
        for(Account account :accounts){
           accountMap.put(account.Adresse_mail__c, account);
        }

        // validate if any leads have the same email address
        for(Lead lnew :trigger.new){     
            
            if(null != lnew.Adresse_mail__c && leadMap.containsKey(lnew.Adresse_mail__c) && (leadMap.get(lnew.Adresse_mail__c).Id !=lnew.Id)){
                Lead ld = leadMap.get(lnew.Adresse_mail__c);

                if(ld.Salutation == null){ld.Salutation = lnew.Salutation;}
                if(ld.FirstName == null){ld.FirstName = lnew.FirstName;}
                ...
            
                updatedLead.add(ld);
                deletedLead.add(lnew);
            }
            
            if(null != lnew.Adresse_mail__c&& accountMap.containsKey(lnew.Adresse_mail__c)){
                Account acc = accountMap.get(lnew.Adresse_mail__c);
                if(acc.Salutation == null){acc.Salutation = lnew.Salutation;}
                if(acc.FirstName == null){acc.FirstName = lnew.FirstName;}
                ...   
                                 
                updatedAccount.add(acc);
                deletedLead.add(lnew);
            }            
        }

        if(updatedLead.size()>0){
            System.debug('mise à jour des pistes');
            update updatedLead;
        }
        
        if(updatedAccount.size()>0){
            System.debug('mise à jour comptes');
            update updatedAccount;
        }

        if(deletedLead.size()>0){
            for (Lead l : deletedLead){
                System.debug('suppression des pistes'+l.Adresse_mail__c);
                MyFutureClass.DeleteLead(l.Id);
            }

        }

    }
}